Upgradeable passive optical network

ABSTRACT

In accordance with the teachings of the present invention, an upgradeable passive optical network is provided. In a particular embodiment, an upgradeable optical network includes an upstream terminal and a distribution node. The upstream terminal is operable to forward a downstream signal comprising traffic in a first wavelength, the traffic in the first wavelength transmitted by a first transmitter at the upstream terminal. The distribution node is communicatively coupled to the upstream terminal and is operable to communicate the downstream signal from the upstream terminal to a first plurality and a second plurality of downstream terminals. The distribution node comprises a primary power splitter configured to create a plurality of copies of the downstream signal for communication to the downstream terminals. The distribution node is configured to facilitate the addition of a wavelength router at a section of the distribution node such that the wavelength router may be added to the distribution node without interrupting the communication of traffic through the network and such that the addition of the wavelength router allows for routing of traffic in a second wavelength transmitted by a second transmitter to be added at the upstream terminal, the traffic destined for the first plurality of downstream terminals; and routing of traffic in a third wavelength transmitted by a third transmitter to be added at the upstream terminal, the traffic destined for the second plurality of downstream terminals.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority under 35 U.S.C. § 119(e)of U.S. Provisional Application entitled “Passive Optical Network usingShared Wavelengths,” Ser. No. 60/729,447 filed Oct. 20, 2005, and ofU.S. Provisional Application entitled “Hybrid Passive Optical NetworkComponents,” Ser. No. 60/756,925 filed Jan. 6, 2006.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to optical networks and, moreparticularly, to an upgradeable passive optical network.

BACKGROUND

In recent years, a bottlenecking of communication networks has occurredin the portion of the network known as the access network. Bandwidth onlonghaul optical networks has increased sharply through new technologiessuch as WDM and transmission of traffic at greater bit rates.Metropolitan-area networks have also seen a dramatic increase inbandwidth. However, the access network, also known as the last mile ofthe communications infrastructure connecting a carrier's central officeto a residential or commercial customer site, has remained at a relativestandstill in terms of affordable bandwidth. The access network thuspresently acts as the bottleneck of communication networks, such as theinternet.

Power-splitting passive optical networks (PSPONs) offer one solution tothe bottleneck issue. PSPONs refer to typical access networks in whichan optical line terminal (OLT) at the carrier's central office transmitstraffic over one or two downstream wavelengths for broadcast to opticalnetwork units (ONUs). An ONU refers to a form of access node thatconverts optical signals transmitted via fiber to electrical signalsthat can be transmitted to individual subscribers. PSPONs address thebottleneck issue by providing greater bandwidth at the access networkthan typical access networks. For example, networks such as digitalsubscriber line (DSL) networks that transmit traffic over coppertelephone wires typically transmit at a rate between approximately 144kilobits per second (KB/s) and 1.5 megabits per second (MB/s).Conversely, Broadband PONs (BPONs), which are example PSPONs, arecurrently being deployed to provide hundreds of megabits per secondcapacity shared by thirty-two users. Gigabit PONs (GPONs), anotherexample of a PSPON, typically operate at speeds of up to 2.5 gigabitsper second (GB/s) by using more powerful transmitters, providing evengreater bandwidth. Other PSPONs include, for example, asynchronoustransfer mode PONs (APONs) and gigabit Ethernet PONs (GEPONs).

Although PSPONs may offer much greater bandwidth than typical accessnetworks such as DSL networks, bandwidth requirements are projected toexceed even the increased capacity offered by typical PSPONs. Forexample, some streaming video and online gaming applications presentlyrequire bit rates of approximately one to ten MB/s, and some IP highdefinition television and video-on-demand systems presently require bitrates of approximately twenty MB/s. Future demands for bandwidth areprojected to be even greater. Thus, a need exists for an access networkthat provides even greater bandwidth.

Another solution to the present bottlenecking issue that would alsosatisfy demand for bandwidth for many years to come is using wavelengthdivision multiplexing passive optical networks (WDMPONs). These networkscomprise access networks in which each ONU receives and transmitstraffic over a dedicated downstream and upstream wavelength,respectively. By transmitting traffic over dedicated wavelengths,WDMPONs dramatically increase network capacity over existing networks(including typical PSPONs). However, WDMPONs tend to be very expensivecompared to PSPONs, the technological risks of deployment of WDMPONs arevery high, and WDMPONs provide much more bandwidth than is presentlydemanded.

SUMMARY

In accordance with the teachings of the present invention, anupgradeable passive optical network is provided. In a particularembodiment, an upgradeable optical network includes an upstream terminaland a distribution node. The upstream terminal is operable to forward adownstream signal comprising traffic in a first wavelength, the trafficin the first wavelength transmitted by a first transmitter at theupstream terminal. The distribution node is communicatively coupled tothe upstream terminal and is operable to communicate the downstreamsignal from the upstream terminal to a first plurality and a secondplurality of downstream terminals. The distribution node comprises aprimary power splitter configured to create a plurality of copies of thedownstream signal for communication to the downstream terminals. Thedistribution node is configured to facilitate the addition of awavelength router at a section of the distribution node such that thewavelength router may be added to the distribution node withoutinterrupting the communication of traffic through the network and suchthat the addition of the wavelength router allows for routing of trafficin a second wavelength transmitted by a second transmitter to be addedat the upstream terminal, the traffic destined for the first pluralityof downstream terminals; and routing of traffic in a third wavelengthtransmitted by a third transmitter to be added at the upstream terminal,the traffic destined for the second plurality of downstream terminals.

Technical advantages of one or more embodiments of the present inventionmay include providing a cost-effective, upgrade path from PowerSplitting Passive Optical Networks (PSPONs), such as APONs, BPONs,GPONs, and GEPONs, to WDMPONs. Particular embodiments may provide apassive optical network with more downstream bandwidth than a typicalPSPON yet avoid the cost and unreliability of WDMPONs by using a morecost-efficient variation of WDMPON features and components fortransmission in the downstream direction. In these embodiments, groupsof ONUs may share one or more of the downstream WDM wavelengths (insteadof each ONU receiving a dedicated wavelength), allowing for relativelycoarse (and thus less expensive) wavelength multiplexing optics in thepassive distribution network. A group of ONUs may comprise less than allof the ONUs corresponding to one OLT.

Particular embodiments may further avoid the cost of WDMPONs byproviding PSPON features and components for transmissions in theupstream direction. These embodiments may allow full re-use of typicaloptical components at each ONU. By reusing optical components, theseembodiments may avoid the cost of new equipment and labor at the time ofupgrade.

Particular embodiments may further avoid the cost of WDMPONs byproviding a wavelength router comprising a filter system as opposed to awavelength router comprising a multiplexer. For example, particularembodiments provide a cascaded filter system. Using cascaded filters andavoiding the use of a costly multiplexer to route downstream wavelengthsmay provide for a less costly network.

Another technical advantage of particular embodiments of the presentinvention includes providing efficient power budgeting between thesignals that are to be split for broadcast to all ONUs and the signalsthat are to be routed to wavelength-sharing ONUs. These embodiments maymanage the power efficiently by allowing the broadcast signals toreceive more power than the routed signals. Thus, the signals in need ofmore power are allotted more power.

Another technical advantage of particular embodiments of the presentinvention includes increasing the number of wavelengths available tocarry traffic by disabling the analog video distribution systemtypically provided in PSPONs and multiplex a number of digital datasignals in the wavelength range around and including the band originallyassigned for analog video broadcast. Particular embodiments may transmittraffic in other bands as well.

Another technical advantage of particular embodiments of, the presentinvention includes providing for an easily upgradeable PSPON. Inparticular embodiments, configuring a PSPON to include components suchas filters coupled to the network through switches may facilitate anupgrade of the PSPON.

It will be understood that the various embodiments of the presentinvention may include some, all, or none of the enumerated technicaladvantages. In addition, other technical advantages of the presentinvention may be readily apparent to one skilled in the art from thefigures, description and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a diagram illustrating an example power splitting PON (PSPON);

FIG. 2 is a diagram illustrating an example WDMPON;

FIG. 3 is a diagram illustrating an example Hybrid PON with WDMdownstream overlay (WDMDO-HPON);

FIG. 4 is a diagram illustrating an example Wavelength Shared Hybrid PON(WS-HPON);

FIG. 5 is a diagram illustrating an example upgradeable PSPON;

FIG. 6 is a diagram illustrating in more detail the example WS-HPON ofFIG. 4;

FIG. 7 is a diagram illustrating an example upgrade to the exampleWS-HPON of FIG. 6;

FIGS. 8A and 8B are diagrams illustrating example remote nodes (RNs)that may be implemented in a WDMDO-HPON;

FIGS. 9A and 9B are diagrams illustrating example RNs that may beimplemented in a WS-HPON;

FIGS. 10A and 10B are diagrams illustrating additional example RNs thatmay be implemented in a WS-HPON;

FIGS. 11A and 11B are diagrams illustrating example components forswitching a filter in or out of the line during a network upgrade; and

FIGS. 12A, 12B, 12C and 12D are diagrams illustrating example componentsfor switching filters in or out of the line during a network upgrade.

DETAILED DESCRIPTION

FIG. 1 is a diagram illustrating an example Power Splitting PassiveOptical Network (PSPON) 10. Typically, PSPONs have been employed toaddress the bottlenecking of communications networks in the portion ofthe network known as the access network. In recent years, bandwidth onlonghaul optical networks has increased sharply through new technologiessuch as wavelength division multiplexing (WDM) and transmission oftraffic at greater bit rates. In addition, metropolitan-area networkshave also seen a dramatic increase in bandwidth. However, the accessnetwork, also known as the last mile of the communicationsinfrastructure connecting a carrier's central office to a residential orcommercial customer site, has not seen as great of an increase inaffordable bandwidth. The access network thus presently acts as thebottleneck of communication networks, such as the internet.

PSPONs address the bottleneck issue by providing greater bandwidth atthe access network than typical access networks. For example, networkssuch as digital subscriber line (DSL) networks that transmit trafficover copper telephone wires typically transmit at a rate betweenapproximately 144 kilobits per second (KB/s) and 1.5 megabits per second(MB/s). Conversely, BPONs are currently being deployed to providehundreds of megabits per second capacity shared by thirty-two users.GPONs, which typically operate at speeds of up to 2.5 gigabits persecond (GB/s) by using more powerful transmitters, provide even greaterbandwidth.

Referring back to PSPON 10 of FIG. 1, PSPON 10 includes an Optical LineTerminal (OLT) 20, optical fiber 30, a Remote Node (RN) 40, and OpticalNetwork Units (ONUs) 50. PSPON 10 refers to typical access networks inwhich an optical line terminal (OLT) at the carrier's central officetransmits traffic over one or two downstream wavelengths for broadcastto optical network units (ONUs). PSPON 10 may be an asynchronoustransfer mode PON (APON), a broadband PON (BPON), a gigabit PON (GPON),a gigabit Ethernet PON (GEPON), or any other suitable PSPON. A featurecommon to all PSPONs 10 is that the outside fiber plant is completelypassive.

OLT 20 (which may be an example of an upstream terminal) may reside atthe carrier's central office (where it may be coupled to a largercommunication network) and includes a transmitter operable to transmittraffic in a downstream wavelength, such as λ_(d), for broadcast to allONUs 50, which may reside at or near customer sites. OLT 20 may alsoinclude a transmitter operable to transmit traffic in a seconddownstream wavelength λ_(v) (which may be added to λ_(d)) for broadcastto all ONUs 50. As an example, in typical GPONs, λ_(v) may carry analogvideo traffic. Alternatively, λ_(v) may carry digital data traffic. OLT20 also includes a receiver operable to receive traffic from all ONUs 50in a time-shared upstream wavelength, λ_(u). In typical PSPONs,downstream traffic in λ_(d) and λ_(v) is transmitted at a greater bitrate than is traffic in λ_(u), as PSPONs typically provide lowerupstream bandwidth than downstream bandwidth. It should be noted that“downstream” traffic refers to traffic traveling in the direction fromthe OLT (or upstream terminal) to the ONUs (or downstream terminals),and “upstream” traffic refers to traffic traveling in the direction fromthe ONUs (or downstream terminals) to the OLT (or upstream terminal).

Optical fiber 30 may include any suitable fiber to carry upstream anddownstream traffic. In certain PSPONs 10, optical fiber 30 may comprise,for example, bidirectional optical fiber. In other PSPONs 10, opticalfiber 30 may comprise two distinct fibers. RN 40 of PSPON 10 (which mayalso generally be referred to as a distribution node) comprises anysuitable power splitter, such as an optical coupler, and connects OLT 20to ONUs 50. RN 40 is located in any suitable location and is operable tosplit a downstream signal such that each ONU 50 receives a copy of thedownstream signal. Due to the split and other possible power losses,each copy forwarded to an ONU has less than 1/N of the power of thedownstream signal received by RN 40, where N refers to the number ofONUs 50. In addition to splitting downstream signals, RN 40 is alsooperable to combine into one signal upstream, time-shared signalstransmitted by ONUs 50. RN 40 is operable to forward the upstream signalto OLT 20.

ONUs 50 (which may be examples of downstream terminals) may include anysuitable optical network unit or optical network terminal (ONT) andgenerally refer to a form of access node that converts optical signalstransmitted via fiber to electrical signals that can be transmitted toindividual subscribers. Subscribers may include residential and/orcommercial customers. Typically, PONs 10 have thirty-two ONUs 50 per OLT20, and thus, many example PONs may be described as including thisnumber of ONUs. However, any suitable number of ONUs per OLT may beprovided. ONUs 50 may include triplexers that comprise two receivers toreceive downstream traffic (one for traffic in λ_(d) and the other fortraffic in λ_(v)) and one transmitter to transmit upstream traffic inλ_(u). The transmission rate of the ONU transmitter is typically lessthan the transmission rate of the OLT transmitter (due to less demandfor upstream capacity than for downstream capacity). Each ONU 50 isoperable to process its designated downstream traffic and to transmitupstream traffic according to an appropriate time-sharing protocol (suchthat the traffic transmitted by one ONU in λ_(u) does not collide withthe traffic of other ONUs in λ_(u)).

In operation, the OLT 20 of a typical PSPON 10 transmits downstreamtraffic destined for one or more of ONUs 50 in λ_(d). OLT 20 may alsotransmit downstream analog video traffic for broadcast to ONUs 50 inλ_(v). Traffic in wavelengths λ_(d) and λ_(v) is combined at OLT 20 andtravels over optical fiber 30 to RN 40. RN 40 splits the downstreamtraffic into a suitable number of copies and forwards each copy to acorresponding ONU. Each ONU receives a copy of the downstream traffic inλ_(d) and λ_(v) and processes the signal. Suitable addressing schemesmay be used to identify which traffic is destined for which ONU 50. EachONU 50 may also transmit upstream traffic in λ_(u) along fiber 30according to a suitable time-sharing protocol (such that upstreamtraffic does not collide). RN 40 receives the upstream traffic from eachONU 50 and combines the traffic from each ONU 50 into one signal. RN 40forwards the signal over fiber 30 to OLT 20. OLT 20 receives the signaland processes it.

Although PSPONs may offer much greater bandwidth than typical accessnetworks such as DSL networks, bandwidth requirements are projected toexceed even the increased capacity offered by typical PSPONs. Forexample, some streaming video and online gaming applications presentlyrequire bit rates of approximately one to ten MB/s, and some IP highdefinition television and video-on-demand systems presently require bitrates of approximately twenty MB/s. Future demands for bandwidth areprojected to be even greater. In the past, network operators have metincreased demand by increasing the transmission rate of transmitters,such as, for example, by upgrading from BPONs to GPONs. However, aswitch to a wavelength division multiplexing PON (WDMPON), in which eachONU would receive and transmit traffic over a dedicated downstream andupstream wavelength, respectively, would dramatically increase networkcapacity and satisfy the demand for bandwidth for many years to come.

FIG. 2 is a diagram illustrating an example WDMPON 100. WDMPON 100 mayinclude any suitable WDMPON (also referred to as WPON) or Dense WDMPON(DWDMPON). WDMPON 100 includes OLT 120, optical fiber 130, RN 140, andONUs 150. Common features of WDMPONs include dedicating at least oneupstream and one downstream wavelength for each ONU. Thus, WDMPONs areoperable to transmit downstream traffic over multiple, dedicatedwavelengths from an OLT, each wavelength corresponding to a particularONU. In addition, each ONU is operable to transmit upstream traffic overa dedicated wavelength, separate from the wavelengths used by the otherONUs 150. Thus, the upstream and downstream bandwidth of WDMPON 100 is Ntimes greater than the bandwidth of a PSPON, where N equals the numberof dedicated wavelengths over which traffic is carried in eachdirection.

Referring back to FIG. 2, OLT 120 of example WDMPON 100 may reside atthe carrier's central office and includes multiple transmitters (equalto the number of ONUs 150), each operable to transmit a dedicateddownstream wavelength, one of λ₁-λ_(n), carrying traffic for acorresponding ONU 150. OLT 120 also includes multiple receivers (equalto the number of ONUs 150), each operable to receive a dedicatedupstream wavelength, one of λ₁-λ_(n), carrying traffic from acorresponding ONU 150. OLT 120 also includes a multiplexer operable tomultiplex the downstream wavelengths transmitted by the transmitters ofOLT 120 and demultiplex the upstream signal (comprising traffic inmultiple wavelengths) that OLT 120 receives from ONUs 150. Afterdemultiplexing the signal, the multiplexer is operable to forward thetraffic in each wavelength to a corresponding receiver in OLT 120. Itshould be noted that λ₁-λ_(n) in the downstream direction may (or maynot) be transmitted at the same wavelengths as λ₁-λ_(n) travelingupstream (despite having similar designation for simplicity of thisdiscussion).

Optical fiber 130 may include any suitable fiber and is operable tocarry upstream and downstream traffic. In certain WDMPONs 100, opticalfiber 130 may comprise, for example, bidirectional optical fiber. Inother WDMPONs 100, optical fiber 130 may comprise two distinct fibers.RN 140 of WDMPON 100 comprises any suitable multiplexer and connects OLT120 to ONUs 150. RN 140 is located in any suitable location and has oneport to receive a downstream signal comprising multiple wavelengths fromOLT 120 and multiple ports (equal to the number of ONUs 150) to forwardtraffic in each wavelength to a corresponding ONU. RN 140 is operable todemultiplex a downstream signal such that each ONU 150 receives trafficover its dedicated downstream wavelength, one of λ₁-λ_(n). RN 140 isalso operable to multiplex upstream traffic carried over λ₁-λ_(n) into asingle upstream signal, the traffic in each wavelength corresponding toone ONU 150. RN 140 is operable to forward the upstream signal to OLT120.

ONUs 150 may include any suitable optical network unit or ONT and mayserve residential and/or commercial customers. Each ONU 150 comprisesone receiver to receive downstream traffic over its dedicated downstreamwavelength from OLT 120. Each ONU 150 also comprises one transmitter totransmit upstream traffic over its dedicated upstream wavelength. EachONU 150 may be capable of transmitting and receiving traffic in anywavelength used in WDMPON 100 such that the wavelengths assigned to thevarious ONUs may be changed without having to change the transmittingand receiving components in the ONUs. An ONU capable of operating inthis fashion is typically referred to as a “colorless” ONU.

In operation, each transmitter in OLT 120 transmits downstream trafficfor a corresponding ONU 150 over a dedicated wavelength, a correspondingone of λ₁-λ_(n). The downstream wavelengths are multiplexed at OLT 150into one signal, and the signal travels over optical fiber 130 to RN140. RN 140 receives the signal and demultiplexes the signal into itsconstituent wavelengths, forwarding the traffic in each wavelength to acorresponding ONU 150. Each ONU 150 receives traffic over the associatedwavelength and processes the traffic. Each ONU 150 may also transmitupstream traffic over a dedicated wavelength, one of λ₁-λ_(n), alongfiber 130. RN 140 receives upstream traffic from all of the ONUs 150carried over these dedicated wavelengths and multiplexes the trafficfrom all of the ONUs 150 into one signal. RN 140 forwards the signalover fiber 130 to OLT 120. OLT 120 receives the signal and demultiplexesit into its constituent wavelengths. The demultiplexer of OLT 120forwards the traffic in each wavelength to a corresponding receiver, andOLT 120 processes the traffic.

As is easily observed in the WDMPON 100 of FIG. 2, WDMPONs dramaticallyincrease bandwidth at the access network by multiplying network capacityby the number of wavelengths carried. However, the increase in bandwidthusing WDMPONs comes at a significant cost. For example, WDMPONs requirehighly stable WDM components to distribute narrowly spaced dedicatedwavelengths (or channels) from the OLT to each ONU and from each ONU tothe OLT. For example, the multiplexer/demultiplexer at RN 140 and thereceivers and transmitter at each ONU 150 must all be precisely tuned.In practice, the density of the channels requires wavelength stabilizedtransmitters and temperature insensitive multiplexers, both of which addsignificantly to the cost of the network. Many WDMPON components(including colorless ONUs) are also expensive and as-of-now unproventechnologies whose reliability has not been determined. Thus, accordingto some estimates, implementing a WDMPON may cost two to five times asmuch as a GPON and WDMPON may be unreliable.

In addition to these high costs, replacing current networks with WDMPONswould also inefficiently increase network capacity beyond present needs.In fact, WDMPONs are expected to exceed demand for many years. Thus,many network operators would prefer to make gradual upgrades fromexisting networks, such that increases in bandwidth (and thus the costof such increases) correspond more closely with increases in consumerdemand. These operators may eventually upgrade to a WDMPON after one ormore intermediary upgrades, thereby incurring the cost of WDMPONbandwidth over a greater period of time and according to consumerdemand.

FIG. 3 is a diagram illustrating an example Hybrid PON 200 with WDMdownstream overlay (WDMDO-HPON). WDMDO-HPON 200 addresses the limitedcapacity issues associated with PSPONs and some of the cost issuesassociated with WDMPONs by providing WDMPON capabilities downstream andPSPON capabilities upstream, thus eliminating the cost and unreliabilityof colorless transmitters at the ONUs. Example WDMDO-HPON 200 may thusbe considered a hybrid PON (or HPON) between a PSPON and a WDMPON.

WDMDO-HPON 200 includes an OLT 220, fiber 230, RN 240, and ONUs 250.WDMDO-HPON 200 provides WDMPON capabilities downstream by using adedicated wavelength, one of λ₁-λ_(n), for each ONU 250. WDMDO-HPON 200provides PSPON capabilities upstream by using one wavelength, λ_(n),time-shared by all ONUs 250.

OLT 220 of WDMDO-HPON 200 (which may be an example of an upstreamterminal) may reside at the carrier's central office and includesmultiple transmitters (equal to the number of ONUs 250), each operableto transmit traffic in a dedicated downstream wavelength, one ofλ₁-λ_(n), carrying traffic for a corresponding ONU 250. As in the caseof GPONs, OLT 220 may also include an additional transmitter to transmitanalog video traffic in λ_(v) for broadcast to all ONUs 250. OLT 220includes a multiplexer operable to multiplex the traffic in thewavelengths transmitted by the multiple transmitters of OLT 220. OLT 220also includes a receiver operable to receive the traffic in the upstreamwavelength, λ_(u), which is time-shared by ONUs 250.

Optical fiber 230 may include any suitable fiber to carry upstream anddownstream traffic. In certain WDMDO-HPONs 200, optical fiber 230 maycomprise, for example, bidirectional optical fiber. In other WDMDO-HPONs200, optical fiber 230 may comprise two distinct fibers.

RN 240 of WDMDO-HPON 200 comprises a multiplexer and a power splitter.The multiplexer is operable to demultiplex dedicated wavelengthsλ₁-λ_(n) and forward traffic in each wavelength, one of λ₁-λ_(n), to acorresponding ONU 250. The power splitter is operable to receive andsplit traffic in downstream wavelength λ_(v) (if applicable) forbroadcast to all ONUs 250. With regard to upstream traffic, the powersplitter of RN 240 is further operable to receive and combine thetraffic transmitted in time-shared λ_(u) from ONUs 250 into one signal.RN 240 is further operable to forward the upstream signal to OLT 220.

ONUs 250 (which may be examples of downstream terminals) may include anysuitable optical network unit or ONT and may serve residential and/orcommercial customers. There may be any suitable number of ONUs, such as,for example, the typical thirty-two ONUs per OLT. Each ONU 250 comprisesone receiver to receive traffic over a dedicated downstream wavelength,one of λ₁-λ_(n), and one receiver to receive traffic over λ_(v) (ifapplicable). Each ONU 250 also comprises one transmitter to transmitupstream traffic over time-shared λ_(u). Each ONU 250 may thus include atriplexer.

In operation, each transmitter in OLT 220 transmits downstream trafficfor a corresponding ONU 250 over a dedicated wavelength, one ofλ₁-λ_(n), and (in certain cases) one transmitter transmits downstreamtraffic to be broadcast to all ONUs 250 over λ_(v). Wavelengths λ₁-λ_(n)and λ_(v) are multiplexed at OLT 220 into one signal, and the signaltravels over optical fiber 230 to RN 240. RN 240 filters the traffic inλ_(v) out of the signal and forwards this wavelength to the powersplitter where it is split for broadcast to all ONUs 250. At themultiplexer, RN 240 demultiplexes the signal comprising the remainingwavelengths (λ₁-λ_(n)) and forwards the traffic in each wavelength, oneof λ₁-λ_(n), to its corresponding ONU 250. Each ONU 250 receives trafficover its dedicated wavelength and processes the traffic. Each ONU 250may also receive and process traffic over λ_(v). In the upstreamdirection, each ONU 250 time-shares use of λ_(u) according to a suitableprotocol. RN 240 receives the time-shared upstream traffic carried overλ_(u) from each of the ONUs 250 and combines the traffic into one signalusing the power splitter. RN 240 forwards the combined signal over fiber230 to OLT 220. OLT 220 receives the signal at its receiver andprocesses the traffic.

Although WDMDO-HPON 200 of FIG. 3 offers a compromise between limitedbandwidth PSPONs and costly WDMPONs, several undesirable features remainin WDMDO-HPONs. For example, WDMDO-HPONs are significantly expensive dueto the costly components necessary to transmit and multiplex largenumbers of wavelengths. As in the case of WDMPONs, WDMDO-HPONs requirehighly stable WDM components to distribute narrowly spaced dedicatedwavelengths from the OLT to each ONU. In practice, the density of thechannels requires wavelength stabilized transmitters and temperatureinsensitive multiplexers, both of which add significantly to the cost ofthe network. In addition to these high costs, replacing current networkswith WDMDO-HPONs would also inefficiently increase network capacitybeyond present needs. Thus, a need exists for an HPON that offers asimple and efficient upgrade from existing PSPONs and that may easilyand efficiently be upgraded (to, for example, a WDMPON).

FIG. 4 is a diagram illustrating an example Wavelength Shared Hybrid PON(WS-HPON) 300. Example WS-HPON 300 comprises OLT 320, optical fiber 330,RN 340, and ONUs 350. Example WS-HPON 300, a hybrid between a PSPON anda WDMPON, provides a more cost-efficient upgrade solution (than aWDMDO-HPON solution) for many network operators. Example WS-HPON 300provides greater downstream capacity without incurring many of the costsof WDMDO-HPON by having groups of two or more ONUs 350 share downstreamWDM wavelengths, thus reducing the need for densely multiplexedwavelengths and the need for highly stable multiplexers andtransceivers.

In the illustrated example, ONUs 350 a-350 n may share λ₁-λ₄. Traffic inλ_(v) is broadcast to all ONUs. It should be noted that any suitablenumber of ONUs may be associated with one OLT. Additionally, anysuitable number of ONUs may share one or more wavelengths in a WS-HPON.Using shared wavelengths in WS-HPON permits the use of less costlyoptics components (such as, for example, low-cost CWDM optics), allowingfor an upgrade in capacity at a lower cost than WDMDO-HPON or WDMPON.

For example, sharing downstream wavelengths in WS-HPON 300 reduces thecost and stability requirements of the multiplexer andtransmitter/receiver components in the network. Due to the sharing ofwavelengths, the spacing between WDM wavelengths may be increased torelax the specifications of wavelength selective elements and to relaxthe requirements for transmitter wavelength stability and temperaturestability of passive components. By using less expensive components toprovide a desired increase in downstream bandwidth, WS-HPON 300 is amuch more attractive upgrade solution for many network operators than isWDMDO-HPON 200. In addition, WS-HPON 300 is also upgradeable to WDMPON,as described below, allowing migration to WDMPON if and when there issufficient demand to justify the cost for the increase in bandwidth.

OLT 320 of WS-HPON 300 (which may be an example of an upstream terminal)may reside at the carrier's central office and comprises fourtransmitters operable to transmit downstream traffic over λ₁-λ₄, whichare to be shared by groups of ONUs 350. OLT 320 may also comprise anadditional transmitter operable to transmit an analog video signal inλ_(v) for broadcast to all ONUs 350. OLT 320 may also comprise amultiplexer operable to multiplex the wavelengths transmitted by thetransmitters of OLT 320. OLT 320 may also comprise a receiver operableto receive upstream traffic in wavelength λ_(u), which is time-shared byONUs 350. It should be noted that although the illustrated embodimentshows only four downstream wavelengths to be shared by ONUs 350, anysuitable number of downstream wavelengths may be transmitted at OLT 320and shared by groups of ONUs 350. In addition, any suitable number ofdownstream wavelengths may be transmitted at OLT 320 and the traffic inthese wavelengths broadcast to all ONUs 350 (and not just the traffic inλ_(v), as illustrated). It should be further noted that traffic in anysuitable number of upstream wavelengths may be received at OLT 320(including traffic in multiple sub-bands of λ_(u)) and an upstreamwavelength need not be time-shared by all ONUs (for example, a separateupstream wavelength may be time-shared by each group of downstream,wavelength-sharing ONUs).

Optical fiber 330 may comprise any suitable fiber to carry upstream anddownstream traffic. In certain WS-HPONs 300, optical fiber 330 maycomprise, for example, bidirectional fiber. In other WS-HPONs 300,optical fiber 330 may comprise two distinct fibers.

RN 340 of WS-HPON 300 may comprise a multiplexer and a power splitter.The multiplexer is operable to demultiplex downstream wavelengths λ₁-λ₄and forward traffic in each of these wavelengths to a correspondinggroup of wavelength-sharing ONUs 350. The power splitter is operable toreceive and split traffic in downstream wavelength λ_(v) (if applicable)for broadcast to all ONUs 350. With regard to upstream traffic, thepower splitter of RN 340 is also operable to receive and combine trafficin time-shared λ_(u) from ONUs 350 into one signal. RN 340 is furtheroperable to forward the upstream signal to OLT 320. It should be notedthat although RN 340 is referred to as a remote node, “remote” refers toRN 340 being communicatively coupled to OLT 320 and ONUs 350 in anysuitable spatial arrangement. A remote node may also generally bereferred to as a distribution node.

ONUs 350 (which may be examples of downstream terminals) may compriseany suitable optical network unit or ONT and may serve residentialand/or commercial customers. There may be any suitable number of ONUs.Each ONU 350 may comprise one receiver to receive traffic over a sharedwavelength, one of λ₁-λ₄, and one receiver to receive traffic over λ_(v)(if applicable). Each ONU 350 may also comprise one transmitter totransmit upstream traffic over time-shared λ_(u). Each ONU 350 may thuscomprise a triplexer.

In operation, the transmitters in OLT 320 transmit downstream trafficover λ₁-λ₄, which are to be shared by groups of ONUs 350, and (incertain cases) one transmitter in OLT 320 transmits downstream trafficto be broadcast to all ONUs 350 over λ_(v). Traffic in wavelengths λ₁-λ₄and λ_(v) is multiplexed at OLT 320 into one signal, and the signaltravels over optical fiber 330 to RN 340. RN 340 filters the traffic inλ_(v) out of the signal and forwards the traffic to the power splitterwhere it is split for broadcast to all ONUs 350. At the multiplexer, RN340 demultiplexes the signal comprising the traffic in the remainingwavelengths (λ₁-λ₄) and forwards the traffic in each wavelength, one ofλ₁-λ₄, to its corresponding group of wavelength-sharing ONUs 350. EachONU 350 receives traffic over one or more of the wavelengths that itshares with other ONUs 350 and processes the traffic (according to asuitable protocol). Each ONU 350 may also receive and process trafficover λ_(v). In the upstream direction, each ONU 350 time-shares use ofλ_(u) according to a suitable protocol. RN 340 receives upstream trafficcarried over time-shared λ_(u) from each of the ONUs 350 and combinesthe traffic into one signal using the power splitter. RN 340 forwardsthe combined signal over fiber 230 to OLT 220. OLT 220 receives thesignal at its receiver and processes the traffic.

Modifications, additions, or omissions may be made to the WS-HPON 300described without departing from the scope of the invention. Thecomponents of the WS-HPON 300 described may be integrated or separatedaccording to particular needs. Moreover, the operations of the WS-HPON300 described may be performed by more, fewer, or other components.

Referring back to the discussion of PSPONs, PSPONs may transmitdownstream traffic over two wavelengths λ_(d) and λ_(v), and upstreamtraffic over time-shared wavelength λ_(u). Many typical PSPONs are noteasily upgradeable to an HPON, such as example WDMDO-HPON 200 or WS-HPON300. Upgrades of typical PSPONs to HPONs require a disruption inservice. For example, in existing PSPONs, fiber cuts are required to addor modify components, disrupting service in existing networks. Thus, aneed exists for an easily upgradeable PSPON.

FIG. 5 is a diagram illustrating an example upgradeable PSPON 400.Example PSPON 400 comprises OLT 420, optical fiber 430, RN 440, and ONUs450. The upgradeability of upgradeable PSPON 400 may be provided for anyPSPON such as those described above in conjunction with PSPON 10 of FIG.1

OLT 420 of example upgradeable PSPON 400 (which may be an example of anupstream terminal) comprises a transmitter 422, transceiver 424comprising transmitter 425 and receiver 426, and filters 428 and 429.Transmitter 422 may comprise any suitable transmitter and is operable totransmit traffic over wavelength λ_(v). Transmitter 422 of typicalPSPONs may transmit analog video traffic (although transmitter 422 mayalternatively transmit digital data traffic). Transmitter 425 oftransceiver 424 may comprise any suitable transmitter operable totransmit traffic over wavelength λ_(d). Transmitter 425 of typicalPSPONs may transmit digital data traffic. Transmitters 422 and 425 maytransmit at any suitable bit rate. Receiver 426 of transceiver 424 maycomprise any suitable receiver operable to receive upstream traffic fromONUs 450 carried over λ_(u). Filter 428 comprises any suitable filteroperable to pass the traffic in λ_(v) and direct the traffic in λ_(d) toRN 440. The traffic in λ_(d) and λ_(v) may also be combined into onesignal at filter 428. Filter 429 comprises any suitable filter operableto pass the traffic in downstream wavelengths λ_(d) and λ_(v) to RN 440and direct the traffic in upstream wavelength λ_(u) to receiver 426.

Optical fiber 430 may comprise any suitable fiber to carry upstream anddownstream traffic. In particular example upgradeable PSPON 400, opticalfiber 430 may comprise bidirectional optical fiber. Alternatively,optical fiber 430 may comprise one fiber for downstream traffic and onefiber for upstream traffic.

RN 440 comprises a filter 442, a lead termination section 446, a primarypower splitter 448, and secondary power splitters 449. RN 440 isconfigured to be easily upgraded to implement a WS-HPON and, eventually,a WDMPON. Filter 442 comprises any suitable filter and may include, forexample, a band splitting filter. Filter 442 is operable to direct thetraffic in downstream wavelengths that are to be split and broadcast byprimary power splitter 448 and pass the traffic in downstreamwavelengths that are to be demultiplexed (once the system is upgraded toa WS-HPON). Filter 442 is also operable to direct the traffic inupstream wavelengths to OLT 420. Before an upgrade from a PSPON, filter442 directs the traffic in λ_(d) and λ_(v) to primary power splitter 448for broadcast, and directs the traffic in λ_(u) from primary powersplitter 448 to OLT 420. Filter 442 may be coupled to a terminationpoint that may terminate the traffic in wavelengths forwarded to it byfilter 442 (although the termination point may be internal to filter 442in alternate example networks). Although the illustrated exampleincludes only one filter 442, example upgradeable PSPONs may compriseany suitable number of filters (and optional switches) to seamlesslyupgrade the network. An example of a set of filters and switches at anRN is described below in conjunction with FIGS. 12A-12D.

Lead termination section 446 of RN 440 allows for an easy upgrade ofnetwork 400 by inserting a wavelength router such as a multiplexer forperforming WDM. Lead termination section 446 comprises a plurality offiber leads. One input fiber lead 447 a is coupled to filter 442, andmultiple output fiber leads 447 b-447 n are coupled to correspondingsecondary power splitters 449. Before an upgrade, leads 447 a-447 n oflead termination section 446 may be configured such that they terminateany signal that they may receive. After an upgrade, leads 447 a-447 nmay be coupled to corresponding multiplexer ports, allowing for easyintegration of a multiplexer (to perform WDM) into the network, asdescribed in more detail below in conjunction with FIG. 6. There may beany suitable number of leads in network 400.

Primary power splitter 448 comprises any suitable power splitter, suchas an optical coupler, and is operable to split a downstream opticalsignal into a suitable number of copies and to combine a plurality ofupstream optical signals into one signal. In the illustrated example,primary power splitter 448 is operable to broadcast the traffic in λ_(d)and λ_(v) by splitting the downstream signal comprising thesewavelengths into four copies (which are then forwarded to secondarypower splitters 449 for eventual receipt by all ONUs 450). Primary powersplitter 448 is also operable to combine upstream traffic transmittedover λ_(u) by ONUs 450 into one signal. Although primary power splitter448 is illustrated as a 1×4 power splitter, primary power splitter 448may comprise any suitable splitter or combination of splitters such thateach ONU 450 receives a copy of the traffic in λ_(d) and λ_(v) and suchthat the upstream signals from ONUs 450 are combined for communicationto OLT 420. A wavelength router and a power splitter may each be membersof a group referred to as distributors.

Each secondary power splitter 449 comprises any suitable power splitter,such as an optical coupler or combination of couplers, operable to splita downstream signal and combine an upstream signal. In the downstreamdirection, each secondary power splitter 449 is operable to receive asignal from primary power splitter 448 and distribute the combinedsignal to a corresponding group of ONUs 450 (in addition, after upgradeto WS-HPON, secondary power splitters 449 serve to combine signalsreceived from primary power splitter 448 and from the multiplexerinserted into lead termination section 446, as described below, and todistribute this combined signal to ONUs 450). Pre-upgrade, secondarypower splitters 449 only receive a signal from primary power splitter448 and do not receive a signal from lead termination section 446. Aftera multiplexer is added to the network, however, secondary powersplitters 449 may receive a signal from primary power splitter 448 andthe multiplexer (coupled to lead termination section 446). In theupstream direction, each secondary power splitter 449 is operable toreceive and combine traffic over time-shared λ_(u) from a correspondinggroup of ONUs 450, split the combined traffic into two copies, anddistribute one copy to lead termination section 446 and one copy toprimary power splitter 448. In the illustrated example, the copy passedto lead termination section 446 is terminated, and the copy passed toprimary power splitter 448 is combined with other upstream signals (fromthe other secondary power splitters 449) and forwarded to filter 442. Itshould be noted that although secondary power splitters 449 arerepresented by 2×4 couplers in the illustrated example, any suitablecoupler (or combination of couplers) may be used.

Each ONU 450 (which may be an example of a downstream terminal) maycomprise any suitable ONU or ONT. In the illustrated example network,ONU 450 comprises a filter 460, a receiver 462, a filter 470, a receiver472, and a transmitter 482. Since each ONU 450 in the illustratedexample comprises two receivers and one transmitter, it may be referredto as a triplexer. However, any suitable number of transmitters and/orreceivers may reside at each ONU. Each filter 460 may comprise anysuitable filter operable to direct traffic in λ_(v) to a correspondingreceiver 462 and to pass traffic in λ_(d) in the downstream directionand λ_(u) in the upstream direction. Each receiver 462 may include anysuitable receiver operable to receive traffic in λ_(v) from acorresponding filter 460 and process the traffic. Each filter 470 maycomprise any suitable filter operable to direct the traffic in λ_(d) toa corresponding receiver 472 and to pass the traffic in λ_(u) in theupstream direction. Each receiver 472 may include any suitable receiveroperable to receive the traffic in λ_(d) from a corresponding filter 470and process the traffic. Each transmitter 482 may comprise any suitabletransmitter operable to transmit the traffic in λ_(u) upstream. ONUs 450may time-share λ_(u) using a suitable communication protocol to avoidcollision of upstream traffic. It should be noted that any suitablenumber of ONUs 450 may be implemented in PSPON 400.

In operation, in the downstream direction, transmitter 422 of OLT 420may transmit downstream traffic (for example, analog video) over λ_(v),and transmitter 425 may transmit downstream traffic (for example,digital data) over λ_(d). Filter 428 receives the two signals and passesthe traffic in λ_(v) and directs the traffic in λ_(d) (thereby combiningthe traffic into one signal) to filter 429. Filter 429 receives thecombined signal comprising the traffic in λ_(v) and λ_(d) and allows thesignal to pass over fiber 430 to filter 442 of RN 440. Filter 442receives the signal comprising the traffic in λ_(d) and λ_(v) anddirects the traffic in both wavelengths to primary power splitter 448.Primary power splitter 448 receives the signal comprising the traffic inλ_(d) and λ_(v), splits the traffic into multiple copies (in thisexample, four copies), and forwards each copy to a correspondingsecondary power splitter 449. Each secondary power splitter 449 receivesa copy of the signal comprising the traffic in λ_(d) and λ_(v), splitsthe copy into multiple additional copies (in this example, fouradditional copies of each copy), and passes each additional copy overfiber 430 to a corresponding ONU 450. Each ONU 450 receives a copy ofthe signal comprising the traffic in λ_(d) and λ_(v) at a correspondingfilter 460. Each filter 460 passes the traffic in λ_(d) to filter 470and directs the traffic in λ_(v) to receiver 462. Filter 470 receivesthe traffic in λ_(d) and directs it to receiver 472. Receivers 462 and472 receive and process the traffic in λ_(v) and λ_(d), respectively.

In the upstream direction, transmitter 482 of each ONU 450 transmitstraffic over time-shared λ_(u). The traffic in λ_(u) passes throughfilters 470 and 460 and over fiber 430 to RN 440. RN 440 receives thetraffic in λ_(u) from each ONU 450 at a corresponding secondary powersplitter 449. Each secondary power splitter 449 splits the signal intotwo copies, and forwards one copy to lead termination section 446 andthe other copy to primary power splitter 448. Lead termination section446 receives a copy of the traffic in λ_(u) from each secondary powersplitter 449 and terminates the traffic. Primary power splitter 448receives a copy of the upstream signal from each secondary powersplitter 449, combines the signals into one signal, and forwards thecombined signal to filter 442. Filter 442 receives the signal comprisingthe traffic in λ_(u) and directs the signal to OLT 420. The signaltravels over fiber 430 to filter 429 of OLT 420. Filter 429 directs thetraffic in λ_(u) to receiver 426. Receiver 426 receives the traffic inλ_(u) and processes it.

Therefore, PSPON 400 operates similarly to PSPON 10 (in that bothbroadcast downstream signals and time-share an upstream signal).However, unlike PSPON 10, PSPON 400 allows for an easy upgrade to aWS-HPON. The optical fiber connecting OLT 420 and ONTs 450 need not becut to perform this upgrade (thereby avoiding network downtime and therisk of contamination of connectors), and the upgrade to a multiplexerat RN 440 and to multiple transmitters and a multiplexer at OLT 420 maybe performed almost completely in-service.

Modifications, additions, or omissions may be made to the PSPON 400described without departing from the scope of the invention. Thecomponents of the PSPON 400 described may be integrated or separatedaccording to particular needs. Moreover, the operations of the PSPON 400described may be performed by more, fewer, or other components. Asexamples only, alternative networks may comprise redundant lines fromthe OLT suitably coupled to the RN, the RN may provide any suitablenumber of outputs to the ONUs, and any suitable number of wavelengthrouters may be added to the RN (making suitable changes to the network).

FIG. 6 is a diagram illustrating in more detail the example WS-HPON ofFIG. 4. The example WS-HPON 500 of FIG. 6 also illustrates an upgradefrom upgradeable PSPON 400 of FIG. 5. As described above, WS-HPONs maybe more attractive upgrade options than WDMDO-HPONs or WDMPONs. Bysharing downstream WDM wavelengths, WS-HPONs provide an increase inbandwidth over PSPONs at a much lower price than WDMDO-HPONs or WDMPONs.

WS-HPON 500 comprises OLT 501, fiber 530, RN 540, and ONUs 550. OLT 501(which may be an example of an upstream terminal) comprises WDMtransmission section 502, upgradeable transmission section 514, PSPONtransceiver 524, and filters 528 and 529. WDM transmission section 502comprises transmitters 504-507, filters 508-511, and multiplexer 512.Each transmitter 504-507 may comprise any suitable transmitter and isoperable to transmit traffic over a corresponding wavelength, λ₁-λ₄,respectively. It should be noted that although example WS-HPON 500 doesnot provide WDM for upstream traffic, and thus new receivers are notneeded in conjunction with new transmitters 504-507, it may beeconomical to implement transceivers (transmitter and receiver) in WDMtransmission section 502 instead of only transmitters (in anticipationof a further upgrade to WDM upstream). The cost of transceivers istypically less than the cost of a separate transmitter and receiver,thus reducing the eventual overall cost to upgrade to a WDMPON. Itshould further be noted that although only four new transmitters areillustrated in example WS-HPON 500, any suitable number of transmitters(or transceivers) may be included.

Each filter 508-511 may comprise any suitable filter and is operable topass the traffic in λ₁-λ₄, respectively. In addition, filter 508 isoperable to direct the traffic in wavelength λ_(v) from transmitter 516of upgradeable transmission section 514 to multiplexer 512 (such thatthe traffic in both λ₁ and λ_(v) is forwarded to multiplexer 512 overthe same fiber). In fact, each filter 508-511 may be coupled toupgradeable transmission section 514 (through a corresponding fiber),allowing for an easy upgrade if more transmitters are added to WS-HPON500, as described below in conjunction with FIG. 7. The fibers couplingfilters 509-511 and upgradeable transmission section 514 do not carry asignal in the illustrated example WS-HPON 500 of FIG. 6, as no newtransmitters have been added to upgradeable transmission section 514.However, filters 509-511 may be operable to direct the traffic in thecorresponding wavelengths associated with potential transmitters addedto upgradeable transmission section 514. After the upgrade, filters509-511 would direct the traffic in the corresponding wavelength fromupgradeable transmission section 514 in a similar manner as filter 508directs the traffic in λ_(v).

Upgradeable transmission section 514 comprises transmitter 516 and a setof three terminating fiber leads. Transmitter 516 comprises any suitabletransmitter and is operable to transmit traffic over λ_(v). Transmitter516 may be the same as transmitter 422 of network 400, and thus λ_(v) ofWS-HPON 500 may be the same as λ_(v) of network 400. Transmitter 516 maytransmit analog video traffic over λ_(v). The three terminating fiberleads may be coupled to new transmitters if new transmitters are addedto upgrade example WS-HPON 500. As discussed above, each terminatinglead is coupled to a corresponding filter, one of filters 509-511. Itshould be noted that although only three fiber leads are illustrated,any suitable number of leads may be added in anticipation of furtherupgrades of WS-HPON 500.

Multiplexer 512 comprises any suitable multiplexer/demultiplexer (andmay be considered a wavelength router) and is operable to combine thetraffic in λ₁-λ₄ and λ_(v) into one signal. In the illustrated example500, multiplexer 512 comprises a cyclic multiplexer operable to receiveand combine the traffic in more than one wavelength through each port(such as the traffic in λ₁ and λ_(v) through the first port). In otherexample networks, multiplexer 512 may be a typical N×1 multiplexeroperable to receive only the traffic in one wavelength through eachport. Where the traffic in a large number of wavelengths is beingmultiplexed, a cyclic multiplexer may prove more cost-efficient than amultiplexer having N ports. Furthermore, a cyclic multiplexer may notneed to be replaced to add more ports (as with typical multiplexers) ifmore transmitters are added to the OLT to upgrade WS-HPON 500. Forexample, as described below in conjunction with FIG. 7, WS-HPON 500 maybe upgraded to transmit traffic in eight downstream wavelengths,including λ₁-λ₄ and additional wavelengths λ₅-λ₈. In such an upgrade,cyclic multiplexer 512 need not be replaced to receive the traffic inthe four additional wavelengths, as the multiplexer's first port mayreceive the traffic in λ₁ and λ₅, the second port may receive thetraffic in λ₂ and λ₆, the third port may receive the traffic in λ₃ andλ₇, and the fourth port may receive the traffic in λ₄ and λ₈. In thecontrasting situation in which non-cyclic multiplexers are used, a 4×1multiplexer would need to be replaced by an 8×1 multiplexer to receivethe traffic in the four additional wavelengths.

PSPON transceiver 524 comprises transmitter 525 and receiver 526.Transmitter 525 may be the same transmitter as transmitter 425 of FIG.5. However, in WS-HPON 500 of FIG. 6, transmitter 525 may no longertransmit traffic over the network, and may be replaced by transmitters504-507 of WDM transmission section 502. Transmitter 525 may, inparticular embodiments, remain in place to provide limited protectionfor failure of the downstream WDM transmitters or to keep the networkoperational at lower bandwidth during particular upgrades of the network(such as, for example, an upgrade of the multiplexer in the RN).Receiver 526 of transceiver 524 may comprise any suitable receiver (andmay be the same as receiver 426 of network 400) and is operable toreceive upstream traffic carried over time-shared λ_(u).

Filter 528 comprises any suitable filter operable to pass the traffic inλ₁-λ₄ and λ_(v) to filter 529. Filter 528 may also be operable to passthe traffic in additional wavelengths to be transmitted in upgrades ofWS-HPON 500 (such as the traffic in λ₅-λ₈ discussed in conjunction withFIG. 7 below). Although illustrated in-line in WS-HPON 500, in otherexample embodiments, filter 528 may be switched out of the line using asuitable switch (such as, for example, the switch illustrated in FIGS.11A and 11B, described below).

Filter 529 comprises any suitable filter operable to pass the traffic inλ₁-λ₄ and λ_(v) over fiber 530 to RN 540. Filter 528 may also beoperable to pass traffic in additional wavelengths to be transmitted inupgrades of WS-HPON 500 (such as traffic in λ₅-λ₈ discussed inconjunction with FIG. 7 below). In addition, filter 529 is operable todirect traffic in upstream wavelength λ_(u) to receiver 526.

Optical fiber 530 may comprise any suitable fiber to carry upstream anddownstream traffic. In certain WS-HPONs 500, optical fiber 530 maycomprise, for example, bidirectional optical fiber. In other WS-HPONs500, optical fiber 530 may comprise two distinct fibers, one carryingdownstream traffic and the other carrying upstream traffic.

RN 540 comprises filter 542, multiplexer 546, primary power splitter548, and secondary power splitters 549. RN 540 is operable to receivethe traffic in λ₁-λ₄ and λ_(v) from OLT 501, filter out and broadcastthe traffic in λ_(v), and demultiplex and forward the traffic in λ₁-λ₄to the ONUs in corresponding groups of wavelength-sharing ONUs 550. RN540 is further operable to receive from ONUs 550 upstream signalscarried over time-shared wavelength λ_(u), combine these signals, andforward the combined traffic in λ_(u) to OLT 501. To reiterate, WS-HPON500 is operable to allow wavelength-sharing among groups of ONUs 550,thereby increasing network capacity while avoiding the costly componentsof a full downstream WDM network such as WDMDO-HPON or WDMPON.

Filter 542 may comprise any suitable filter operable to receive a signalcomprising traffic in λ₁-λ₄ and λ_(v), pass the traffic in λ₁-λ₄ tomultiplexer 546, and direct the traffic in λ_(v) to primary powersplitter 548. Although filter 542 in the illustrated example includesonly one filter, filter 542 may comprise any suitable number of filters(coupled to optional switches) to facilitate an upgrade of the network(described below in conjunction with FIGS. 12A-12D). In the upstreamdirection, filter 542 is operable to receive the traffic in λ_(u) anddirect it toward OLT 501.

Multiplexer 546 may include any suitable multiplexer/demultiplexer (andmay be considered a wavelength router) and is operable to receive thesignal comprising the traffic in λ₁-λ₄ and demultiplex the signal.Although in the illustrated example, multiplexer 546 is a 1×4multiplexer, in alternative networks, multiplexer 546 may have anysuitable number of ports. Also, in alternative networks, multiplexer 546may comprise two or more separate multiplexers receiving downstreamsignals from one or more upstream sources and forwarding the trafficdownstream such that ONUs share wavelengths. In the illustrated examplenetwork, multiplexer 546 may comprise a cyclic multiplexer (in whicheach port is operable to carry traffic in more than one wavelength). Inthe downstream direction, each output port of multiplexer 546 isoperable to forward the traffic in a corresponding one of λ₁-λ₄ to acorresponding secondary power splitter 549. In alternative embodiments,the traffic in each wavelength may pass to a different secondary powersplitter than that illustrated, the traffic in more than one wavelengthmay pass to a secondary power splitter, and/or multiplexer 546 mayreceive, multiplex, and pass traffic in more than four downstreamwavelengths. It should be noted that to upgrade PSPON 400 to WS-HPON500, multiplexer 546 may be added to the lead termination section 446 ofupgradeable PSPON network 400 (with input lead 447 a coupled to themultiplexer's input port and output leads 447 b-447 n coupled to themultiplexer's output ports).

In the upstream direction, multiplexer 546 may be operable to receiveand terminate the traffic in λ_(u), as ONUs 550 of example WS-HPON 500time-share λ_(u) (and do not transmit traffic over multiple upstreamwavelengths). Alternatively, multiplexer 546 may forward this traffic tofilter 542 for suitable termination (where termination may be performedinternally or externally). However, if WS-HPON 500 is upgraded toprovide WDM upstream, multiplexer 546 may be operable to receive trafficin multiple upstream wavelengths from ONUs 550 and multiplex the trafficin these wavelengths, forwarding them to OLT 501.

Primary power splitter 548 may comprise any suitable power splitter andmay be primary power splitter 448 of network 400. In the illustratedexample WS-HPON, primary power splitter 548 is operable to receive thetraffic in λ_(v) and split the traffic into four copies. The power ofeach copy may be less than one-fourth of the power of the originalsignal λ_(v). Primary power splitter 548 is operable to forward eachcopy to a corresponding secondary power splitter 549. In the upstreamdirection, primary power splitter 548 is operable to receive traffictransmitted by ONUs 550 over time-shared λ_(u) from secondary powersplitters 549 and combine this traffic into one signal. Primary powersplitter 548 forwards the upstream signal to OLT 501. Primary powersplitter 548 thus broadcasts the traffic in λ_(v) in the downstreamdirection and combines traffic over time-shared λ_(u) in the upstreamdirection. Although primary power splitter 548 is illustrated as a 1×4power splitter, any suitable power splitter may be used.

Each secondary power splitter 549 may comprise any suitable powersplitter, such as an optical coupler, operable to receive a signal fromprimary power splitter 548 and a signal from multiplexer 546, combinethe two signals into one signal, split the combined signal into asuitable number of copies, and forward each copy to the ONUs in acorresponding wavelength-sharing group of ONUs 550 (each group ofwavelength-sharing ONUs shares one of λ₁-λ₄ in the downstreamdirection). In the upstream direction, each secondary power splitter 549is operable to receive traffic transmitted at λ_(u) from each ONU 550 ofa corresponding group of ONUs 550 and combine the traffic from each ONU550 into one signal. Each secondary power splitter 549 is operable tosplit the combined upstream traffic into two copies and forward one copyto primary power splitter 548 and one copy to multiplexer 546. The copyforwarded to primary power splitter 548, as described above, is combinedwith other traffic from other ONUs 550 transmitted over time-sharedλ_(u). The copy forwarded to multiplexer 546 may be blocked or forwardedto filter 542 for suitable termination. Although secondary powersplitters 549 are illustrated as 2×4 couplers in example WS-HPON 500,secondary power splitters 549 may be any suitable coupler or combinationof couplers (such as a 2×2 coupler coupled to two 1×2 couplers).Secondary power splitters 549 may split or combine any suitable numberof signals.

Each ONU 550 (which may be an example of a downstream terminal) maycomprise any suitable ONU or ONT and may be the same as ONU 450 of FIG.5. ONU 550 of FIG. 6 comprises a filter 560, receiver 562, filter 570,receiver 572, and transmitter 582. Each filter 560 may comprise anysuitable filter and may be the same as filter 460 of network 400. Eachfilter 560 is operable to direct traffic in wavelength λ_(v) (forexample, analog video traffic) to receiver 562. Filter 560 is furtheroperable to pass the traffic in the corresponding one of λ₁-λ₄ receivedat the ONU 550 to filter 570 and to pass the traffic in λ_(u) to RN 540in the upstream direction. Receiver 562 may comprise any suitablereceiver and may be the same as receiver 462 of network 400. Receiver562 is operable to receive the traffic transmitted in λ_(v) and processthe traffic. Each filter 570 may comprise any suitable filter and may bethe same as filter 470 of network 400. Each filter 570 is operable toreceive the traffic in a corresponding one of λ₁-λ₄ and direct it toreceiver 572. Filter 570 is further operable to pass the traffic inupstream wavelength λ_(u) to corresponding filter 560 in the upstreamdirection. Receiver 572 may comprise any suitable receiver and may bethe same as receiver 472 of network 400. Receiver 572 is operable toreceive the traffic transmitted in a corresponding one of λ₁-λ₄ andprocess the traffic. Receiver 572 may be operable to receive traffic inany one of λ₁-λ₄, providing flexibility in assigning (or re-assigning)an ONU 550 to a particular wavelength-sharing group. Each transmitter582 may comprise any suitable transmitter and may be the same astransmitter 482 of network 400. Transmitter 582 is operable to transmittraffic over λ_(u) in the upstream direction, applying a suitableprotocol to time-share λ_(u) with the other ONUs 550. It should be notedthat although four ONUs are illustrated as being part of a group of ONUsin WS-HPON 500, any suitable number of ONUs may be part of a groupsharing a downstream wavelength. In addition, there may be multiplegroups each sharing a different a downstream wavelength (as is the casein the illustrated example). It should also be noted that any suitablenumber of ONUs 550 may be implemented in the network. It should furtherbe noted that ONUs 550 may be adjusted in an alternative upgrade totransmit traffic over sub-bands of λ_(u) (which may be combined by PS548 of RN 540) to be received by receivers corresponding to transmitters504-507 (in which case filter 529 would be taken out of the line). In analternative upgrade, each group of ONUs sharing a wavelength maytransmit upstream traffic in a separate wavelength (and multiplexer 546may multiplex these wavelengths at the RN, and receivers 504-507 mayreceive these wavelengths).

In operation, transmitters 504-507 and (analog video) transmitter 516 ofOLT 501 may transmit traffic over λ₁-λ₄ and λ_(v), respectively. Filters508-511 receive the traffic in λ₁-λ₄, respectively, and pass thesignals. Filter 508 also receives and directs the traffic in λ_(v),combining it with the traffic in λ₁. As discussed above, WS-HPON 500 isconfigured to be easily upgraded to include additional transmitters atupgradeable transmission section 514 of OLT 501. After receiving thetraffic in their corresponding wavelengths, filters 508-511 forward thecorresponding signals to multiplexer 512. Multiplexer 512, which mayinclude, for example, a cyclic multiplexer, combines the traffic in thefive wavelengths into one signal and forwards the signal to filter 528.As discussed above, filter 528 may be in-line or may be switched out ofthe line, in which case the downstream signal would pass directly tofilter 529. If filter 528 remains in-line, filter 528 receives thedownstream signal and allows the signal to pass to filter 529. Filter529 receives the signal and allows the signal to pass, forwarding thesignal over optical fiber 530 to RN 540.

Filter 542 of RN 540 receives the signal and directs the traffic in(e.g., analog video) wavelength λ_(v) to primary power splitter 548,allowing the traffic in λ₁-λ₄ to pass to multiplexer 546. Primary powersplitter 548 receives the traffic in λ_(v) and splits it into a suitablenumber of copies. In the illustrated embodiment, primary power splitter548 splits the traffic in λ_(v) into four copies, and forwards each copyto a corresponding secondary power splitter 549. Multiplexer 546receives the signal comprising the traffic in λ₁-λ₄ and demultiplexesthe signal into its constituent wavelengths. Multiplexer 546 thenforwards the traffic in each wavelength along a corresponding fiber suchthat each secondary power splitter 549 receives the traffic in acorresponding one of λ₁-λ₄. Each secondary power splitter 549 thusreceives traffic in a corresponding one of λ₁-λ₄ from multiplexer 546and a copy of traffic in λ_(v) from primary power splitter 548, combinesthe two wavelengths into one signal, and splits the signal into asuitable number of copies. In the illustrated embodiment, each secondarypower splitter 549 splits the signal into four copies. In this way, thetraffic (e.g., analog video) in wavelength λ_(v) is broadcast to allONUs 550 and a corresponding one of λ₁-λ₄ is transmitted to and sharedby one or more groups of ONUs 550. It should be noted again that thegroups of ONUs sharing a wavelength may be different than thoseillustrated in FIG. 6, and groups of wavelength-sharing ONUs may sharemore than one WDM wavelength in alternative networks.

After secondary power splitters 549 split the signal comprising thetraffic in a corresponding one of λ₁-λ₄ and the traffic in λ_(v) intofour copies, secondary power splitters 549 forward each copy over fiber530 such that the ONUs 550 coupled to the secondary power splitter 549receive a copy. Filter 560 of each ONU 550 receives the signal anddirects the traffic in λ_(v) to receiver 562, which then processes thetraffic carried over λ_(v). Filter 560 passes the corresponding one ofλ₁-λ₄ to filter 570. Filter 570 receives the traffic in thecorresponding one of λ₁-λ₄ and directs the traffic to receiver 572 whichthen processes the traffic. Again, since each ONU 550 in a group mayshare one of λ₁-λ₄ with other ONUs 550 in the group, ONUs 550 must applya suitable addressing protocol to process downstream trafficappropriately (i.e., to determine which portion of the traffictransmitted in the corresponding wavelength is destined for which ONU550 in a group).

In the upstream direction, transmitter 582 of each ONU 550 transmitstraffic over λ_(u). Filters 570 and 560 receive the traffic in λ_(u) andpass the traffic. The signal travels over fiber 530 to RN 540. Eachsecondary power splitter 549 of RN 540 receives traffic over time-sharedλ_(u) and combines the traffic from each ONU 550 in the correspondinggroup of ONUs 550. Again, since each ONU 550 transmits traffic overupstream wavelength λ_(u), ONUs 550 must adhere to a suitable protocolto time-share λ_(u) such that traffic from multiple ONUs does notcollide. After receiving and combining traffic over λ_(u) into onesignal, each secondary power splitter 549 splits the signal into twocopies, forwarding one copy to multiplexer 546 and one copy to primarypower splitter 548. As discussed above, multiplexer 546 of examplenetwork 500 may block λ_(u) or forward λ_(u) to filter 542 for suitabletermination (internal or external to filter 542). Primary power splitter548 receives traffic over λ_(u) from each secondary power splitter 549,combines the traffic, and forwards the traffic to filter 542. Filter 542receives the combined traffic in λ_(u) and directs the traffic towardOLT 501. Fiber 530 carries the traffic in λ_(u) to filter 529 of OLT501. Filter 529 receives the traffic in λ_(u) and directs the traffictoward receiver 526. Receiver 526 receives the traffic and processes it.

Modifications, additions, or omissions may be made to the exampleWS-HPON 500 described without departing from the scope of the invention.The components of the example WS-HPON 500 described may be integrated orseparated according to particular needs. Moreover, the operations of theexample WS-HPON 500 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers may be added to the RN (making suitablechanges to the network).

As discussed above, WS-HPON 500 is an attractive PSPON upgrade for manynetwork operators because it increases network capacity by introducingwavelength division multiplexing of downstream traffic without modifyingONUs 550, avoiding the cost of replacing components at each ONU and ofcreating substantial disruption in service. WS-HPON 500 is also a moreattractive option than other HPONs such as WDMDO-HPON. By having groupsof ONUs share downstream wavelengths, WS-HPON 500 avoids the need formore costly optics components (that must operate under stricterconditions and that are untested and unreliable). WS-HPON 500 thusallows for a desired increase in downstream bandwidth through WDM at areasonable and efficient cost. In addition, WS-HPON 500 is configured toallow additional upgrades as demand for bandwidth increases (forexample, adding additional downstream wavelengths and eventuallyupgrading to full WDMPON) without a substantial disruption to thenetwork.

FIG. 7 is a diagram illustrating an example upgrade to the exampleWS-HPON of FIG. 6. Upgraded WS-HPON 600 comprises additionaltransmitters that allow traffic to travel over additional wavelengths,thereby increasing the capacity of the network. As discussed above inconjunction with FIG. 6, example WS-HPON 500 is configured to be easilyupgradeable to include additional transmitters. These additionaltransmitters may replace transmitter 516 and may transmit in and aroundthe same band used by transmitter 516.

Example upgraded WS-HPON 600 comprises OLT 601, optical fiber 630, RN640, and ONUs 650. OLT 601 (which may be an example of an upstreamterminal) comprises first transmission section 602, second transmissionsection 613, transceiver 624, and filters 628 and 629. Firsttransmission section 602 comprises transmitters 604-607, filters608-611, and multiplexer 612. Transmitters 604-607 may be the sametransmitters as transmitters 504-507 of FIG. 6; thus, these transmitterswill not be described again. Filters 608-611 may be the same as filters508-511 of FIG. 6; thus, these filters will also not be described again.However, it should be noted that each of filters 608-611 now receivestraffic in a corresponding wavelength from a transmitter of secondtransmission section 613. Thus, in the illustrated embodiment, filters608-611 receive λ₁-λ₄, respectively, and λ₅-λ₈, respectively.Multiplexer 612 may be the same as multiplexer 512 of FIG. 6; thus,multiplexer 612 will not be described. In particular example networks inwhich multiplexer 612 is a cyclic multiplexer (as illustrated),multiplexer 612 receives more than one wavelength at each port. Inexample networks in which multiplexer 612 is not a cyclic multiplexer,filters 608-611 may not be needed (as each transmitter would transmittraffic directly to an input port of multiplexer 612). It should benoted that although eight transmitters, four filters, and onemultiplexer are illustrated in example WS-HPON 600, any suitablecomponents may be used to forward WDM signals comprising any suitablenumber of wavelengths to RN 640.

Second transmission section 613 comprises additional transmitters totransmit traffic over additional wavelengths. In the illustrated exampleWS-HPON, additional transmitters 614-617 transmit traffic over λ₅-λ₈,respectively. In particular example networks, the traffic may be digitaldata traffic, replacing the analog video signal transmitted bytransmitter 516 of example WS-HPON 500. It should be noted that inaddition to replacing the video analog transmitter in particular examplenetworks, transmitters 614-617 may also transmit in and around the sameband that the analog video signal was transmitted such that receivers562 of network 500 may be used as receivers 662 of network 600. Itshould further be noted that although example WS-HPON 600 does notprovide WDM for upstream traffic, and thus new receivers are not neededin conjunction with new transmitters 614-617, it may be most economicalto implement transceivers for second transmission section 613 instead ofonly transmitters (in anticipation of a further upgrade to WDMupstream).

Transceiver 624 may be the same as transceiver 524 of FIG. 6; thus,transceiver 624 will not be described again. Filters 628 and 629 may bethe same as filters 528 and 529 of FIG. 6; however, it should be notedthat filters 628 and 629 are operable to pass λ₁-λ₈ in the downstreamdirection, and filter 629 is operable to direct λ_(u) toward receiver626. In addition, as discussed in conjunction with filter 528 of FIG. 6,filter 628 may be switched out of the line. As discussed in conjunctionwith filter 529 of network 500, filter 629 may be removed in alternativenetworks to allow for WDM upstream.

Optical fiber 630 may comprise any suitable fiber and is operable tocarry upstream and downstream traffic. In particular networks, opticalfiber 630 may comprise, for example, bidirectional optical fiber. Inalternative networks, optical fiber 630 may comprise two differentfibers, one for upstream traffic and one for downstream traffic.

RN 640 comprises filter 642, multiplexer 646, primary power splitter648, and secondary power splitters 649. Filter 642 may comprise anysuitable filter operable to pass λ₁-λ₈ to multiplexer 646. Filter 642,in particular embodiments, no longer directs any of the downstreamwavelengths toward primary power splitter 648. In example WS-HPON 600,filter 642 continues to direct upstream wavelength λ_(u) toward OLT 601.Thus, filter 642 may be the same as filter 542 of FIG. 6 in someembodiments; however, in alternative embodiments, filter 642 may replacefilter 542 if filter 542 is not operable to pass λ₁-λ₈.

Multiplexer 646 may be the same as multiplexer 546 of FIG. 6 and thuswill not be described again. However, it should be noted thatmultiplexer 646 now demultiplexes λ₅-λ₈ in addition to λ₁-λ₄. In theillustrated embodiment, multiplexer 646 is a 1×4 cyclic multiplexer, andforwards λ₁ and λ₅ through the first output port, λ₂ and λ₆ through thesecond output port, λ₃ and λ₇ through the third output port, and λ₄ andλ₈ through the fourth output port. In alternative embodiments,multiplexer 646 may be any other suitable multiplexer, and may, forexample, forward only one wavelength per output port. In addition,multiplexer 646 may have any suitable number of ports and notnecessarily the number illustrated in FIG. 7. It should be noted that insome instances, a cyclic multiplexer may be most cost-efficient, as itmay not need to be replaced if the number of downstream wavelengths isincreased and exceeds the number of multiplexer ports (such as theincrease due to the upgrade illustrated in FIG. 7).

Primary power splitter 648 may be the same as primary power splitter 548of FIG. 6 and thus will not be described again. However, it should benoted that, in the illustrated example, primary power splitter 648 nolonger receives downstream wavelengths since downstream wavelengthsλ₁-λ₈ are now all routed by multiplexer 646. However, primary powersplitter 648 continues to receive and combine upstream traffic overλ_(u) and forward the combined signal to OLT 601. In alternativeembodiments, if a transmitter at OLT 601 transmits traffic for broadcastto all ONUs, primary power splitter 648 is operable to receive and splitthat signal into copies for distribution to all ONUs 650.

Secondary power splitters 649 may be the same as secondary powersplitters 549 of FIG. 6 and thus will not be described again. However,it should be noted that in the downstream direction, in the illustratedexample network, each secondary power splitter 649 receives traffic in acorresponding one or more of λ₁-λ₈ from multiplexer 646 and no signalfrom primary power splitter 648. In alternative embodiments, there maybe a different number of secondary power splitters, and each secondarypower splitter may receive different, less, or additional wavelengths.Each secondary power splitter 649 is operable to forward a correspondingone or more of λ₁-λ₈ to a corresponding group of ONUs 650. In this way,secondary power splitters 649 allow for wavelength-sharing among ONUs.

ONUs 650 (which may be examples of downstream terminals) may be the sameas ONUs 550 of FIG. 6, and thus will not be described again. However, itshould be noted that in the illustrated embodiment, in addition toreceiving traffic in a corresponding one of λ₁-λ₄, each ONU 650 mayreceive traffic in a corresponding one of λ₅-λ₈. Filter 660 is thusoperable to direct a corresponding one of λ₅-λ₈ to receiver 662.Receiver 662 is operable to receive traffic in λ₅-λ₈ (and there is noneed to replace receiver 562 during the upgrade to network 600) sinceλ₅-λ₈ are in and around the same band as λ_(v). Thus, receiver 662 isoperable to process the signal carried in λ₅-λ₈. It should further benoted that the electronics downstream of receiver 662 may be convertedto process digital data (as opposed to analog traffic) in some examplenetworks. However, even if these electronics are not converted, they maynonetheless be operable to process digital data carried in λ₅-λ₈suitably.

In operation, transmitters 604-607 and 614-617 of OLT 601 transmitdownstream traffic over λ₁-λ₈, respectively. Filters 608-611 receiveλ₁-λ₄, respectively, and λ₅-λ₈, respectively, combine their respectivesignals, and pass the signals to multiplexer 612. For example, filter608 receives λ₁ and λ₅, combines the two signals into one signal, andpasses the signal to multiplexer 612. Multiplexer 612, which mayinclude, for example, a cyclic multiplexer, combines the eightwavelengths into one signal and forwards the signal to filter 628. Asdiscussed above, filter 628 may be in-line or may be switched out of theline, in which case the signal would pass directly to filter 629. Iffilter 628 remains in-line, filter 628 receives the signal frommultiplexer 612 and passes the signal to filter 629. Filter 629 receivesthe signal and allows the signal to pass, forwarding the signal overoptical fiber 630 to RN 640.

Filter 642 of RN 640 receives the downstream signal and allows thetraffic in λ₁-λ₈ to pass to multiplexer 646. In the illustrated examplenetwork, traffic in all downstream wavelengths is to be routed tomultiplexer 646 and not split by primary power splitter 648 for generalbroadcast. Thus, primary power splitter 648 of the illustrated examplereceives none of the traffic in the downstream wavelengths. However, inalternative example networks, power splitter 648 may continue to receivetraffic in one or more downstream wavelengths for general broadcast.

Multiplexer 646 receives the signal comprising traffic in λ₁-λ₈ anddemultiplexes the signal. In the illustrated example network,multiplexer 646 is a cyclic multiplexer, and thus multiplexer 646forwards traffic in a corresponding pair of wavelengths from each outputport to a corresponding secondary power splitter 649. For example,multiplexer 646 forwards λ₁ and λ₅ through its first port, λ₂ and λ₆through its second port, λ₃ and λ₇ through its third port, and λ₄ and λ₈through its fourth port. Thus, multiple groups of ONUs share multiplewavelengths for reception of downstream traffic. Each secondary powersplitter 649 receives a corresponding signal from multiplexer 646 (andno signal from primary power splitter 648) and splits the signal into asuitable number of copies. In the illustrated embodiment, each 2×4secondary power splitter 649 splits the signal it receives frommultiplexer 646 into four copies and forwards each copy to acorresponding ONU 650 over fiber 630. It should be noted again that inalternative networks, the ONUs sharing a WDM wavelength may be differentthan those illustrated in FIG. 7.

Each ONU 650 receives a copy forwarded by a corresponding secondarypower splitter 649. Filter 660 of each ONU 650 receives the signal anddirects the traffic in the corresponding one of λ₅-λ₈ to receiver 662,which then processes the signal. Filter 660 passes traffic in thecorresponding one of λ₁-λ₄ to filter 670. Filter 670 directs the trafficin the corresponding one of λ₁-λ₄ to receiver 672, which then processesthe signal. Again, since each ONU 650 in a group of wavelength-sharingONUs may share a corresponding one of λ₁-λ₄ and a corresponding one ofλ₅-λ₈ with other ONUs in the group, ONUs 650 must apply a suitableprotocol to process the traffic appropriately.

The illustrated, example upgraded WS-HPON 600 has been shown anddescribed as using the increased bandwidth provided by the upgrade todouble the bandwidth of each existing group of wavelength-sharing ONUs650. However, in alternative example networks, the increased bandwidthmay be used to increase the number of groups of wavelength-sharing ONUs650 (such as, for example, from four groups to eight groups) or toallocate bandwidth more dynamically among the ONUs in each group (suchas, for example, having one ONU in a group receive and process trafficin two wavelengths and having another ONU in the group receive thetraffic in the two wavelengths but process the traffic in only one ofthe wavelengths). In some of such cases, suitable changes to the RN 640may be necessary.

In the upstream direction, transmitter 682 of each ONU 650 transmitstraffic over λ_(u). Filters 670 and 660 receive and pass the traffic inλ_(u). The signal travels over fiber 630 to RN 640. Each secondary powersplitter 649 of RN 640 receives traffic over time-shared λ_(u) andcombines the traffic from each ONU 650 in the corresponding group ofONUs 650. Again, since each ONU 650 transmits traffic over upstreamwavelength λ_(u), ONUs 650 must adhere to a suitable protocol totime-share λ_(u) such that upstream traffic from ONUs 650 does notcollide. After receiving and combining the upstream traffic from theONUs 650 in the corresponding group into one signal, each secondarypower splitter 649 splits the combined signal into two copies,forwarding one copy to multiplexer 646 and one copy to primary powersplitter 648. As discussed above, multiplexer 646 of WS-HPON 600terminates λ_(u) or may allow λ_(u) to pass to filter 642 for suitabletermination (since upstream traffic is still not WDM in the illustratedexample). Primary power splitter 648 receives traffic in λ_(u) from eachsecondary power splitter 649, combines the traffic, and forwards thetraffic to filter 642. Filter 642 receives the traffic in λ_(u) anddirects it toward OLT 601. Fiber 630 carries the traffic in λ_(u) tofilter 629 of OLT 601, which directs the traffic toward receiver 626.Receiver 626 receives the signal and processes it.

As demand for upstream bandwidth increases beyond current capacity,network operators may choose to upgrade their WS-HPON (such as WS-HPON600) to provide WDM upstream as well as downstream. Network operatorsmay upgrade to WDM upstream by providing every ONU within each group ofwavelength-sharing ONUs with a transmitter(s) such that the ONUs of thatgroup transmit traffic in the same upstream wavelength(s). Thus, forexample, the ONUs in group one may transmit traffic over upstreamwavelength λ₁ (and thus suitably time-share transmission over thiswavelength), the ONUs in group two may transmit traffic over upstreamwavelength λ₂ (and thus suitably time-share transmission over thiswavelength), and so on. In alternative example networks, the ONUs of onegroup may transmit traffic over a first plurality of wavelengths, theONUs of another group may transmit traffic over a second plurality ofwavelengths, and so on. In fact, any suitable number of upstreamwavelengths may be transmitted by the ONUs of each group (and members ofone group may share an additional wavelength with members of anothergroup). In an upgrade of network 600 (in which the OLT has eightreceivers to receive eight upstream wavelengths), the ONUs of each groupof ONUs may transmit traffic in two wavelengths (resulting in trafficbeing transmitted in eight total upstream wavelengths). As newtransmitters may be added at the ONUs, the λ_(u) transmitters of theONUs may be replaced with these transmitters.

At the RN, the fibers on either side of the primary power splitter maybe terminated, and filter 642 may be taken out of the line (as describedfurther below in conjunction with FIG. 12D). The primary power splittermay no longer be used in either the downstream or upstream directionsince the RN's multiplexer would suitably demultiplex downstream trafficand multiplex upstream traffic. For example, the multiplexer may receivea copy of traffic forwarded by each secondary power splitter at acorresponding port, combine the traffic received at its ports into onesignal, and forward the combined signal to the OLT. Alternatively, innetworks providing WDM upstream over sub-bands (such as, for example,sub-bands of λ_(u)), the upstream WDM signals may be received at powersplitter 648 and forwarded to the OLT, and the upstream signalsforwarded to multiplexer 646 may be suitably terminated.

At the OLT, the multiplexer/demultiplexer may demultiplex the upstreamsignal into its constituent wavelengths. If the demultiplexer is acyclic multiplexer, each upstream port may output two or morewavelengths, and filters may further separate the traffic. Receiverscoupled to downstream transmitters may receive the upstream traffic in acorresponding wavelength and process the traffic. In the alternativenetwork in which WDM traffic is transmitted in sub-bands of λ_(u), thefilter coupling the λ_(u) receiver may be taken out of the line,allowing the sub-bands to pass to the WDM upstream receivers in the OLT.Thus, downstream and upstream WDM would be provided. To upgrade thesystem further to WDMPON, additional transmitters and receivers would beadded at the OLT and at the ONUs (corresponding to the number ofadditional transmitters and receivers needed to provide a dedicateddownstream and upstream wavelength for each ONU) and suitable rewiringwould take place at the RN.

Modifications, additions, or omissions may be made to the exampleWS-HPON 600 described without departing from the scope of the invention.The components of the WS-HPON 600 described may be integrated orseparated according to particular needs. Moreover, the operations of theWS-HPON 600 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers may be added to the RN (making suitablechanges to the network).

FIGS. 8A and 8B are diagrams illustrating example RNs that may beimplemented in a WDMDO-HPON. Example RN 700 of FIG. 8A comprises an RNtypically used in WDMDO-HPONs, and example RN 800 of FIG. 8B comprisesan improved RN for WDMDO-HPONs. Example RN 700 of FIG. 8A comprises afilter 710, a power splitter 720, a wavelength router 730, and opticalfilters 740.

As discussed above in conjunction with FIG. 3, the RN of typicalWDMDO-HPONs splits a signal, λ_(v), for broadcast to all ONUs (which maybe an analog video signal) and demultiplexes the other dedicatedwavelengths, λ₁-λ_(n) (such that the traffic in one of λ₁-λ_(n) passesto a corresponding ONU). Thus, in RN 700, filter 710 is operable todirect λ_(v) toward power splitter 720 to be broadcast to all ONUs.Filter 710 is also operable to pass all other dedicated wavelengths,λ₁-λ_(n), to wavelength router 730. Power splitter 720 is operable toreceive the traffic in λ_(v) and to split the signal suitably. In theillustrated embodiment, power splitter 720 splits the traffic in λ_(v)into n copies (where n equals the number of ONUs) and forwards a copy toeach of filters 740. Wavelength router 730 may include any suitablewavelength router, such as, for example, a multiplexer. Wavelengthrouter 730 is operable to receive the signal comprising dedicatedwavelengths λ₁-λ_(n) from filter 710 and demultiplex the signal into itsconstituent wavelengths, each wavelength corresponding to one ONU.Wavelength router 730 is further operable to forward each wavelength toa corresponding filter, one of 740 a-740 n. Each filter 740 is operableto receive traffic in a corresponding one of λ₁-λ_(n) from wavelengthrouter 730 as well as a copy of the traffic in λ_(v) from power splitter720, combine these two signals, and forward the combined signal to acorresponding ONU.

In operation, filter 710 of RN 700 receives a signal comprising trafficin dedicated wavelengths, λ₁-λ_(n), and wavelength λ_(v) to be broadcastto all ONUs. Filter 710 directs the traffic in λ_(v) toward powersplitter 720 and passes the traffic in λ₁-λ_(n) to wavelength router730. Power splitter 720 receives the traffic in λ_(v), splits it into ncopies, and forwards a copy to each filter 740. Wavelength router 730receives a signal comprising traffic in λ₁-λ_(n) from filter 710,demultiplexes the signal into its constituent wavelengths, and forwardsthe traffic in each wavelength to a corresponding filter 740. Eachfilter 740 receives traffic in a corresponding dedicated wavelength (oneof λ₁-λ_(n)) as well as a copy of the traffic in λ_(v), combines thesetwo signals into one signal, and forwards the combined signal to acorresponding ONU.

Due to the number of filters it uses, RN 700 may be quite costly. Thus,a need exists for an RN that uses more cost-efficient opticalcomponents. RN 800 of FIG. 8B provides an alternative to RN 700 of FIG.8A and uses more cost-efficient optical components such as couplersinstead of filters, mitigating the power loss disadvantage of couplersby using asymmetric couplers.

RN 800 comprises filter 810, power splitter 820, wavelength router 830,and couplers 840. Filter 810, power splitter 820, and wavelength router830 may be the same as filter 710, power splitter 720, and wavelengthrouter 730, respectively, of RN 700, and thus will not be describedagain. Each coupler 840 may include any suitable coupler operable tocombine a copy of the traffic in λ_(v) from power splitter 820 and thetraffic in a corresponding one of λ₁-λ_(n) from wavelength router 830.Each coupler 840 may be a 2×1 asymmetric coupler, such that the powerloss at each coupler is greater for one input signal than for the other.In RN 800, each coupler 840 may be configured such that the power lossassociated with the coupler is greater for the traffic in thecorresponding one of λ₁-λ_(n) than for the traffic in λ_(v). In such acase, in the upstream direction, the copy of the traffic traveling tothe power splitter would receive more of the power than the copy of thetraffic traveling to the wavelength router, a beneficial result in thecase of an upstream wavelength time-shared by all ONUs. In thedownstream direction, this asymmetrical distribution of power lossallows each copy from power splitter 820 (which has much less power thanthe original traffic in λ_(v) before the power split) to receive less ofthe power loss associated with coupler 840 than the routed traffic inthe corresponding one of λ₁-λ_(n) (which did not lose as much powerthrough wavelength router 830 as the traffic that was split by powersplitter 820). Thus, as an example only, the net power of the traffic inthe dedicated wavelength and the net power of the traffic in the splitwavelength may be substantially the same after each receives itsassociated power loss at the asymmetric coupler. As another example, thetraffic in λ_(v) may be associated with ten percent of the power loss,and the corresponding one of λ₁-λ_(n) may be associated with ninetypercent of the power loss. Power loss may in fact be managed in anysuitable manner.

By managing power loss in an asymmetric manner, RN 800 mitigates thedisadvantage of using couplers instead of using filters as in RN 700.This disadvantage is that couplers typically produce greater power lossthan filters; thus, traffic in λ_(v), already weakened by power splitter820, would be weakened more by a downstream coupler than by a downstreamfilter, resulting in a signal of even lower quality. By using asymmetriccouplers 840, however, RN 800 distributes greater power loss associatedwith coupler 840 to the routed traffic (which may suitably accommodatesuch power loss) than to the split traffic (which may not be able tosuitably accommodate more power loss), thereby mitigating thedisadvantage of using couplers instead of filters. By mitigating thepower loss disadvantage of using couplers instead of filters and byusing less expensive asymmetric couplers than more expensive filters, RN800 offers an improvement to RN 700 of typical WDMDO-HPON networks. Itshould be noted that although in the illustrated example, the traffic inλ_(v) is associated with ten percent of the power loss, and the trafficin the corresponding one of λ₁-λ_(n) is associated with ninety percentof the power loss, any suitable ratio may be used.

In operation, filter 810 of RN 800 receives a signal comprising thetraffic in dedicated wavelengths, λ₁-λ_(n), and the traffic in an analogvideo or other wavelength λ_(v) to be broadcast to all ONUs. Filter 810directs the traffic in λ_(v) toward power splitter 820 and passes thetraffic in λ₁-λ_(n) to wavelength router 830. Power splitter 820receives the traffic in λ_(v), splits it into n copies, and forwards acopy to each coupler 840. Wavelength router 830 receives the signalcomprising the traffic in λ₁-λ_(n) from filter 810, demultiplexes thesignal into its constituent wavelengths, and forwards the traffic ineach wavelength to a corresponding coupler 840. Each coupler 840receives the traffic in the corresponding wavelength as well as a copyof the traffic in λ_(v) from power splitter 820 and combines these twosignals into one signal such that power loss is shared asymmetrically.In the illustrated example RN 800, the copy of the traffic in λ_(v)(which is the weaker of the two signals) receives less of the power lossassociated with coupler 840 than the traffic in the routed wavelength(which is the stronger of the two signals). In the illustrated example,the traffic in the routed wavelength receives 90% of the power lossassociated with coupler 840 and the copy of the traffic in λ_(v)receives 10% of the power loss. Each coupler 840 then forwards thecombined signal to a corresponding ONU.

In the upstream direction, couplers 840 may receive time-shared trafficin λ_(u). Each coupler may split the traffic in λ_(u) that it receives,forwarding one (weaker) copy to wavelength router 830 and one (stronger)copy to power splitter 820. Wavelength router 830 may terminate thecopies it receives or may forward the copies to filter 810 for suitabletermination. Power splitter 820 may combine the copies it receives andforward the combined signal to filter 810. Filter 810 may receive thecombined signal and direct it to the OLT. Filter 810 may also terminateupstream traffic suitably (such as, for example, in an upgrade to WDMupstream, terminating traffic directed to it by power splitter 820).

RN 800 uses less costly components (couplers versus filters) than doesRN 700. In addition, downstream signals forwarded by RN 800 havecomparable optical characteristics as those forwarded by RN 700. This isso because the disadvantage associated with using couplers instead offilters (that of an increase in power loss and thus decrease in signalperformance) is mitigated by the use of asymmetric couplers. Asdescribed above, asymmetric couplers shift power loss to the strongersignal (the signal received from the wavelength router) and away fromthe weaker signal (the signal received from the power splitter), thusmanaging power loss efficiently.

Modifications, additions, or omissions may be made to the example RN 800described without departing from the scope of the invention. Thecomponents of the example RN 800 described may be integrated orseparated according to particular needs. Moreover, the operations of theexample RN 800 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers may be added to the RN (making suitablechanges to the network).

FIGS. 9A and 9B are diagrams illustrating example RNs 900 and 1000 thatmay be implemented in a WS-HPON. RN 900 of FIG. 9A comprises filter 910,primary power splitter 920, wavelength router 930, filters 940, andsecondary power splitters 950. Filter 910 comprises any suitable filteroperable to direct analog video traffic in λ_(v) (or any wavelength ortype of signal to be broadcast to all ONUs) to primary power splitter920 and pass traffic in λ₁-λ₄ (or traffic in any wavelength which is tobe routed) to wavelength router 930. Filter 910 may also receive andforward upstream traffic to the OLT. Primary power splitter 920comprises any suitable power splitter. Primary power splitter 920 isoperable to receive a signal from filter 910, split the signal into asuitable number of copies, and forward each copy to a correspondingfilter 940. As an example, in the illustrated example RN, primary powersplitter 920 splits the signal in λ_(v) (which may be an analog videosignal) into four copies. In the upstream direction, primary powersplitter 920 is operable to combine traffic forwarded by filters 940 andforward the traffic to filter 910.

Wavelength router 930 may comprise any suitable wavelength router suchas, for example, a multiplexer, operable to receive a signal from filter910 and suitably route the signal's constituent wavelengths tocorresponding filters 940. In the upstream direction, in an exampleWS-HPON not using WDM upstream, wavelength router 930 may terminate anyupstream signal it receives or forward the upstream signal to filter 910for suitable termination.

Each filter 940 may comprise any suitable filter or other suitablecombining element operable to receive a copy of traffic in λ_(v) fromprimary power splitter 920 and the traffic in a corresponding one ofλ₁-λ₄ from wavelength router 930. Each filter 940 is further operable tocombine the two signals into one signal and forward the signal to acorresponding secondary power splitter 950. In the upstream direction,each filter 940 is operable to direct time-shared λ_(u) to powersplitter 920.

Each secondary power splitter 950 may comprise any suitable coupler orcombination of couplers, such as, for example, a 1×2 coupler asillustrated. Each secondary power splitter 950 is operable to receive asignal from a corresponding filter 940, split the signal into a suitablenumber of copies, and forward each copy. In a WS-HPON such as WS-HPON500, each copy may be forwarded to another corresponding power splitter(such as a 1×2 splitter), and each resulting copy may be forwarded to anONU. In alternative WS-HPONs, any suitable number of power splitters(such as a single 1×4 secondary power splitter) may be used to spliteach signal into a suitable number of copies.

In operation, filter 910 receives a downstream signal comprising trafficin λ₁-λ₄ and λ_(v), directs the traffic in λ_(v) to primary powersplitter 920, and passes the traffic in λ₁-λ₄ to wavelength router 930.Primary power splitter 920 receives the signal comprising traffic inλ_(v) from filter 910, splits the signal into a suitable number ofcopies, and forwards each copy to a corresponding filter 940. Wavelengthrouter 930 receives a signal comprising the traffic in λ₁-λ₄ from filter910 and routes the signal's constituent wavelengths to correspondingfilters 940. Each filter 940 receives the traffic in a corresponding oneof λ₁-λ₄ as well as a copy of the traffic in λ_(v), combines the twosignals into one signal, and forwards the signal to a correspondingsecondary power splitter 950. Each secondary power splitter 950 splitsthe signal into a suitable number of copies and forwards each copy. In aWS-HPON such as WS-HPON 500, each copy may be forwarded to anothercorresponding power splitter (such as a 1×2 splitter), and eachresulting copy may be forwarded to an ONU. In alternative WS-HPONs, anysuitable number of power splitters (such as a single 1×4 secondary powersplitter) may be used to split the signal into a suitable number ofcopies.

In the upstream direction, each secondary power splitter 950 may receiveand combine traffic in time-shared λ_(u) and forward the combined signalto a corresponding filter 940. Each filter 940 may receive thecorresponding combined signal and direct it to power splitter 920. Powersplitter 920 may receive the combined signals from filters 940, combinethem, and forward them to filter 910. Filter 910 may direct the signalforwarded by power splitter 920 to the OLT.

Modifications, additions, or omissions may be made to the example RN 900described without departing from the scope of the invention. Thecomponents of the example RN 900 described may be integrated orseparated according to particular needs. Moreover, the operations of theexample RN 900 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers may be added to the RN (making suitablechanges to the network).

RN 900 uses filters 940 and secondary power splitters 950 to combinesignals from primary power splitter 920 and wavelength router 930, tosplit the combined signal into a suitable number of copies, and toforward the copies downstream. Using both filters and secondary powersplitters, however, adds considerable expense to RN 900. Example RN 1000of FIG. 9B illustrates an RN that uses 2×2 secondary power splitters(instead of filters and 1×2 secondary power splitters) to combinesignals from the primary power splitter and the wavelength router, splitthe combined signal into a suitable number of copies, and forward thecopies downstream, saving the large expense associated with usingfilters. Example RN 1000, besides costing less than RN 900, alsoproduces signals with comparable optical characteristics as those in RN900. This is due to the comparable power loss associated with an M×Npower splitter (used in RN 900 in conjunction with filter 940) and anN×N power splitter (used in RN 1000).

RN 1000 comprises filter 1010, primary power splitter 1020, wavelengthrouter 1030, and secondary power splitters 1040. Filter 1010, primarypower splitter 1020, and wavelength router 1030 may be the same asfilter 910, primary power splitter 920, and wavelength router 930,respectively, of WR 900, and thus will not be described again. Secondarypower splitters 1040 of RN 1000 act as an alternative to the combinationof filters 940 and secondary power splitters 950 of RN 900. Secondarypower splitters 1040 may comprise any suitable power splitters or othersuitable combining elements, such as optical couplers. In theillustrated example RN 1000, each secondary power splitter 1040comprises a 2×2 coupler. In alternative embodiments, secondary powersplitters 1040 may comprise any suitable N×N coupler (terminating anyunused ports). N×N couplers are used because these couplers may performtwo functions: combining N signals and splitting the combined signalinto N copies. Furthermore, the power loss associated with an N×Ncoupler is comparable to that associated with a 1×N coupler. Thus, thefunction of two devices such as the costly filter and the 1×N secondarypower splitter in RN 900 is performed by the N×N secondary powersplitter, and the power loss between the two RNs is comparable. In theillustrated embodiment, each 2×2 secondary power splitter 1040 isoperable to receive traffic in λ_(v) from primary power splitter 1020and routed traffic in a corresponding one of λ₁-λ₄ from wavelengthrouter 1030, combine the traffic in the two signals into one signal,split the traffic in the signal into two copies, and forward each copyto one or more ONUs. In a WS-HPON such as WS-HPON 500, each copy may beforwarded to another corresponding power splitter (such as a 1×2splitter), and each resulting copy may be forwarded to an ONU.Alternatively, the secondary power splitters may be 4×4 couplers (withtwo input ports terminated in each coupler), each 4×4 coupler forwardinga copy of the combined traffic to an ONU. In either alternative, anysuitable number of power splitters may be used to split each signal intoa suitable number of copies. In the upstream direction, each powersplitter 1040 is operable to receive traffic in time-shared λ_(u) from acorresponding group of wavelength-sharing ONUs, combine the traffic,split the combined traffic into two copies, and forward one copy towavelength router 1030 (which may then be suitably terminated) and onecopy to power splitter 1020 (which may then be combined with othertraffic in λ_(u) and forwarded to filter 1010).

In operation, filter 1010 receives a signal comprising traffic in λ₁-λ₄and λ_(v), directs traffic in λ_(v) to primary power splitter 1020, andallows traffic in λ₁-λ₄ to pass to wavelength router 1030. Primary powersplitter 1020 splits the signal comprising traffic in λ_(v) into asuitable number of copies, and forwards each copy to a correspondingsecondary power splitter 1040. Wavelength router 1030 demultiplexes thesignal comprising the traffic in λ₁-λ₄, and forwards the traffic in eachwavelength to a corresponding one or more secondary power splitters1040. Each secondary power splitter 1040 receives a copy of the trafficin λ_(v) from primary power splitter 1020 and the traffic in acorresponding one of λ₁-λ₄ from wavelength router 1030, combines the twosignals into one signal, splits the signal into a suitable number ofcopies, and forwards each copy downstream. In a WS-HPON such as WS-HPON500, each copy may be forwarded to another corresponding power splitter(such as a 1×2 splitter), and each resulting copy may be forwarded to anONU. Alternatively, the secondary power splitters may be 4×4 couplers(with two input ports terminated in each coupler), each 4×4 couplerforwarding a copy of the combined traffic to an ONU. In eitheralternative, any suitable number of power splitters may be used to spliteach signal into a suitable number of copies. As discussed above, bycombining the functions of the filter and secondary power, splitter ofRN 900 into one N×N power splitter, RN 1000 provides a less-costlyalternative and provides signals with comparable opticalcharacteristics.

In the upstream direction, each secondary power splitter 1040 mayreceive traffic in time-shared λ_(u) from a corresponding group ofwavelength-sharing ONUs, combine the traffic, split the combined trafficinto two copies, and forward one copy to wavelength router 1030.Wavelength router 1030 may then suitably terminate the copy or forwardthe copy to filter 1010 for suitable termination (internal or externalto the filter). Each secondary power splitter 1040 forwards a secondcopy to primary power splitter 1020. Primary power splitter 1020 mayreceive a copy from each secondary power splitter 1040, combine thetraffic into one signal, and forward the combined signal to filter 1010.Filter 1010 may receive the combined signal and direct it to the OLT.Filter 1010 may also terminate upstream traffic suitably (such as, forexample, in an upgrade to WDM upstream, terminating traffic directed toit by power splitter 1020).

Modifications, additions, or omissions may be made to the example RN1000 described without departing from the scope of the invention. Thecomponents of the example RN 1000 described may be integrated orseparated according to particular needs. Moreover, the operations of theexample RN 1000 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers may be added to the RN (making suitablecorresponding changes to the network).

FIGS. 10A and 10B are diagrams illustrating additional example RNs thatmay be implemented in a WS-HPON. Example RNs 1100 and 1300 illustrateRNs that route wavelengths without using a multiplexer, providing apotentially less costly RN solution, while producing signals withcomparable optical characteristics. Example RN 1100 of FIG. 10Acomprises a filter 1110, a primary power splitter 1120, filters1130-1160, and secondary power splitters 1170. Filter 1110 may be thesame as filter 710 or filter 910 of FIGS. 8A and 9A, respectively. Thus,filter 1110 will not be described again. Primary power splitter 1120 maycomprise any suitable power splitter operable to receive a signal fromfilter 1110 and split the signal into a suitable number of copies. As anexample, in the illustrated example RN, primary power splitter 1120splits an analog video signal or other traffic in λ_(v) into fourcopies. Primary power splitter 1120 forwards each copy to acorresponding filter, one of filters 1130-1160 (but may alternativelyforward each copy to a corresponding secondary power splitter 1170). Inthe upstream direction, power splitter 1120 may be operable to receivetraffic in λ_(u) from filters 1130-1160 (or alternatively, fromsecondary power splitters 1170). If the network provides WDMcapabilities upstream, filters 1130-1160 may be operable to forward theupstream traffic to filter 1110 (and may bypass power splitter 1120).

Filters 1130-1160 may comprise any suitable filters (such as, forexample, four-port, thin-film filters) or suitable combinations offilters (such as, for example, suitable combinations of three-portfilters). Filter 1130 is operable to receive a signal comprisingmultiple wavelengths from filter 1110 and a copy of the signal split byprimary power splitter 1120. In the illustrated example, filter 1130 isoperable to receive a signal from filter 1110 comprising traffic inλ₁-λ₄, pass traffic in λ₁ to secondary power splitter 1170 a, and directtraffic in λ₂-λ₄ to filter 1140. Filter 1130 is further operable toreceive a copy of traffic in λ_(v) from primary power splitter 1120 anddirect it to secondary power splitter 1170 a. Thus, filter 1130 mayforward traffic in λ₁ and λ_(v) to secondary power splitter 1170 a.

Similarly, filter 1140 is operable to receive the traffic in λ₂-λ₄ fromfilter 1130 and a copy of the traffic in λ_(v) from primary powersplitter 1120. Filter 1140 is further operable to pass the traffic in λ₂to secondary power splitter 1170 b, direct traffic in λ_(v) to secondarypower splitter 1170 b, and direct traffic in λ₃ and λ₄ toward filter1150. Thus, filter 1140 may forward traffic in λ₂ and λ_(v) to secondarypower splitter 1170 b.

Likewise, filter 1150 is operable to receive traffic in λ₃ and λ₄ fromfilter 1140 and a copy of traffic in λ_(v) from primary power splitter1120. Filter 1150 is further operable to pass traffic in λ₃ to secondarypower splitter 1170 c, direct traffic in λ_(v) to secondary powersplitter 1170 c, and direct traffic in λ₄ toward filter 1160. Thus,filter 1140 may forward traffic in λ₃ and λ_(v) to secondary powersplitter 1170 c.

In addition, filter 1160 is operable to receive traffic in λ₄ fromfilter 1150 and a copy of traffic in λ_(v) from primary power splitter1120. Filter 1160 is further operable to pass traffic in λ₄ and directtraffic in λ_(v) to secondary power splitter 1170 d. Any remainingwavelengths may be terminated. Because each of filters 1130-1160 passesa wavelength and directs the remaining wavelengths (if any) todownstream filters (if any), the filters may be referred to as a“cascaded” filters. It should be noted that any suitable number ofwavelengths may be forwarded by one or more filters to a secondary powersplitter. It should further be noted that there may be any suitablenumber of chains of cascaded filters in an RN.

Each secondary power splitter 1170 is operable to receive a signal froma corresponding filter (one of filters 1130-1160), split the signal intoa suitable number of copies, and forward each copy to a suitable numberof ONUs. A suitable number of additional couplers may be used to furthersplit the signals such that each ONU receives a suitable signal. Itshould be noted that although RN 1100 has been described with referenceto a WS-HPON, RN 1100 may also be implemented in WDMDO-HPONs and inother PONs using WDM (by, for example, removing secondary powersplitters 1170 and/or adding additional cascaded filters for routing anyadditional wavelengths). It should further be noted that, in analternative example RN, copies of the broadcast traffic in λ_(v) may beforwarded by primary power splitter 1120 to secondary power splitters1170 instead of being forwarded to the filters themselves.

In operation, filter 1110 receives a signal comprising traffic in λ₁-λ₄and λ_(v), directs traffic in λ_(v) toward primary power splitter 1120and passes traffic in the remaining wavelength, λ₁-λ₄, to filter 1130.Primary power splitter 1120 receives the signal comprising the trafficin λ_(v) and splits the signal into a suitable number of copies. In theillustrated example RN, primary power splitter 1120 splits the signalinto four copies and forwards each copy to a corresponding one offilters 1130-1160. Filter 1130 receives the signal comprising thetraffic in λ₁-λ₄ from filter 1110 and a copy of the traffic in λ_(v)from primary power splitter 1120. Filter 1130 passes the traffic in λ₁to secondary power splitter 1170 a, directs the traffic in λ₂-λ₄ towardfilter 1140, and directs the traffic in λ_(v) toward secondary powersplitter 1170 a. Filter 1140 receives the signal comprising the trafficin λ₂-λ₄ from filter 1130 and a copy of the traffic in λ_(v) fromprimary power splitter 1120. Filter 1140 passes traffic in λ₂ tosecondary power splitter 1170 b, directs traffic in λ₃ and λ₄ towardfilter 1150, and directs traffic in λ_(v) toward secondary powersplitter 1170 b. Filter 1150 receives the signal comprising traffic inλ₃ and λ₄ from filter 1140 and a copy of traffic in λ_(v) from primarypower splitter 1120. Filter 1150 passes traffic in λ₃ to secondary powersplitter 1170 c, directs traffic in λ₄ toward filter 1160, and directstraffic in λ_(v) toward secondary power splitter 1170 c. Filter 1160receives the signal comprising traffic in λ₄ from filter 1150 and a copyof traffic in λ_(v) from primary power splitter 1120. Filter 1160 passestraffic in λ₄ and directs traffic in λ_(v) to secondary power splitter1170 d. Filter 1160 may suitably terminate any remaining wavelengths.Although in the illustrated example RN 1100 there are no other filtersin the cascade, alternative RNs may have more or less wavelengths toroute (and thus filters). Each secondary power splitter 1170 receives asignal comprising the traffic in λ_(v) and the traffic in acorresponding one of λ₁-λ₄ and splits the signal into a suitable numberof copies. A suitable number of additional couplers may further splitthe signals such that each ONU receives a suitable signal.

In the upstream direction, secondary power splitters 1170 may receiveupstream traffic in time-shared λ_(u), combine the traffic, and forwardthe combined signal to filters 1130-1160. Each filter 1130-1160 mayreceive a corresponding combined signal and forward the signal toprimary power splitter 1120. Primary power splitter 1120 receives andcombines the combined signals and forwards the resulting signal tofilter 1110. Filter 1110 receives the signal and forwards it to the OLT.In the case of WDM upstream, the traffic in the multiple upstreamwavelengths may circumvent power splitter 1120.

Example RN 1100 may perform wavelength routing without the use of acostly multiplexer. By not using a multiplexer, RN 1100 may be lesscostly (while providing comparable performance) than RNs that do use amultiplexer. It should be noted that although RN 1100 has been describedwith reference to WS-HPON, RN 1100 may also be implemented in otherHPONs such as WDMDO-HPON (by, for example, removing secondary powersplitters 1170 and adding a suitable number of additional cascadingfilters for routing any additional wavelengths).

Modifications, additions, or omissions may be made to the example RN1100 described without departing from the scope of the invention. Thecomponents of the example RN 1100 described may be integrated orseparated according to particular needs. Moreover, the operations of theexample RN 1100 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers and/or chains of cascaded filters may beadded to the RN (making suitable changes to the network).

FIG. 10B illustrates an additional example RN that may be implemented ina WS-HPON. Example RN 1300 of FIG. 10C does not use a filtercorresponding to filter 1110 of RN 1100 and thus may cost less than RN1100 of FIG. 10A. However, by foregoing the use of a filtercorresponding to filter 1110 of RN 1100, example RN 1300 may require aless tolerant (and thus possibly more expensive) thin-film filterdesign. In addition, by splitting traffic in all downstream wavelengths(instead of splitting only traffic that is to be broadcast), RN 1300 mayintroduce higher insertion loss for routed signals than RN 1100,producing signals with less desirable optical characteristics than thoseproduced by RN 1100.

Example RN 1300 comprises primary power splitter 1320, filters1330-1360, and secondary power splitters 1370. Primary power splitter1320 comprises any suitable power splitter operable to receive adownstream signal comprising multiple wavelengths. Primary powersplitter 1320 is further operable to split the signal into a suitablenumber of copies. In the illustrated embodiment, the downstream signalcomprises traffic in λ₁-λ₄ and λ_(v), where the traffic in λ₁-λ₄ is tobe routed by RN 1300 to corresponding groups of wavelength-sharing ONUsand traffic in λ_(v) is to be broadcast by RN 1300 to all ONUs. Inalternative example RNs, the number of wavelengths may differ from thatillustrated. In such RNs, any suitable number of wavelengths (andcorresponding optical components) may be provided. In the upstreamdirection, primary power splitter 1320 is operable to receive traffic intime-shared λ_(u) from filters 1330-1360, combine the traffic, andforward the combined signal to the OLT.

Filter 1330 may comprise any suitable filter. Filter 1330 is operable toreceive a copy of the traffic in λ₁-λ₄ and λ_(v) from primary powersplitter 1320. Filter 1330 is further operable to direct the traffic inλ₁ and λ_(v) toward secondary power splitter 1370 a. Filter 1330 isfurther operable to suitably terminate (internally or externally) thetraffic in the remaining wavelengths, λ₂-λ₄.

Filter 1340 may comprise any suitable filter. Filter 1340 is operable toreceive a copy of the traffic in λ₁-λ₄ and λ_(v) from primary powersplitter 1320. Filter 1340 is further operable to direct the traffic inλ₂ and λ_(v) toward secondary power splitter 1370 b. Filter 1340 isfurther operable to suitably terminate (internally or externally) thetraffic in the remaining wavelengths, λ₁ and λ₃-λ₄.

Filter 1350 may comprise any suitable filter. Filter 1350 is operable toreceive a copy of the traffic in λ₁-λ₄ and λ_(v) from primary powersplitter 1320. Filter 1350 is further operable to direct the traffic inλ₃ and λ_(v) toward secondary power splitter 1370 c. Filter 1350 isfurther operable to suitably terminate (internally or externally) thetraffic in the remaining wavelengths, λ₁-λ₂ and λ₄.

Filter 1360 may comprise any suitable filter. Filter 1360 is operable toreceive a copy of the traffic in λ₁-λ₄ and λ_(v) from primary powersplitter 1320. Filter 1360 is further operable to direct the traffic inλ₄ and λ₅ toward secondary power splitter 1370 d. Filter 1360 is furtheroperable to suitably terminate (internally or externally) the remainingwavelengths, λ₁-λ₃.

Each secondary power splitter 1370 may comprise any suitable powersplitter, such as a coupler, operable to receive a signal from acorresponding one of filters 1330-1360. Each signal comprises traffic inλ_(v) and traffic in a corresponding one of λ₁-λ₄. Each secondary powersplitter 1370 is further operable to split the received signal into asuitable number of copies and forward each copy to one or morewavelength-sharing ONUs.

In operation, primary power splitter 1320 receives a downstream signalcomprising traffic in λ₁-λ₄ and λ_(v), splits the signal into fourcopies, and forwards each copy to a corresponding one of filters1330-1360. Filter 1330 receives a copy from primary power splitter 1320,directs the traffic in λ₁ and λ_(v) to secondary power splitter 1370 a,and suitably terminates (internally or externally) the traffic in theremaining wavelengths (λ₂-λ₄). Filter 1340 receives a copy from primarypower splitter 1320, directs the traffic in λ₂ and λ_(v) to secondarypower splitter 1370 b, and suitably terminates (internally orexternally) the traffic in the remaining wavelengths (λ₁ and λ₃-λ₄).Filter 1350 receives a copy from primary power splitter 1320, directsthe traffic in λ₃ and λ_(v) to secondary power splitter 1370 c, andsuitably terminates (internally or externally) the traffic in theremaining wavelengths (λ₁-λ₂ and λ₄). Filter 1360 receives a copy fromprimary power splitter 1320, directs the traffic in λ₄ and λ_(v) tosecondary power splitter 1370 d, and suitably terminates (internally orexternally) the traffic in the remaining wavelengths (λ₁-λ₃). Eachsecondary power splitter 1370 receives a signal from a corresponding oneof filters 1330-1360, splits the signal into a suitable number ofcopies, and forwards each copy to one or more wavelength-sharing ONUs.Additional couplers may further split the signals such that each ONUreceives a suitable signal.

In the upstream direction, secondary power splitters 1370 may receiveupstream traffic in time-shared λ_(u), combine the traffic, and forwardthe combined signal to filters 1330-1360. Each filter 1330-1360 mayreceive a corresponding combined signal and forward the signal toprimary power splitter 1320, suitably terminating other traffic. Primarypower splitter 1320 receives and combines the combined signals andforwards the resulting signal to the OLT.

Thus, example RN 1300 may perform wavelength routing without the use ofa costly multiplexer. Furthermore, example RN 1300 does not use a filtercorresponding to filter 1110 of example RN 1100. Although not using sucha filter may provide a less costly RN, using filters 1330-1360 which maybe less tolerant (and thus more costly) than those of RN 1100 may offsetany cost savings. In addition, the optical characteristics of routedtraffic in wavelengths λ₁-λ₄ may be less desirable than the routedtraffic in RN 1100.

It should be noted that although four wavelengths are illustrated inexample RN 1300, any suitable number of WDM wavelengths may be routed(using a suitable number of filters and couplers). It should also benoted that although RN 1300 has been described with reference to aWS-HPON, RN 1300 may also be implemented in other HPONs using WDM suchas WDMDO-HPON (by, for example, removing secondary power splitters 1370and adding suitable filters for routing any additional wavelengths).

Modifications, additions, or omissions may be made to the example RN1300 described without departing from the scope of the invention. Thecomponents of the example RN 1300 described may be integrated orseparated according to particular needs. Moreover, the operations of theexample RN 1300 described may be performed by more, fewer, or othercomponents. As examples only, alternative networks may compriseredundant lines from the OLT suitably coupled to the RN, the RN mayprovide any suitable number of outputs to the ONUs, and any suitablenumber of wavelength routers and/or chains of filters may be added tothe RN (making suitable corresponding changes to the network).

FIGS. 11A and 11B are diagrams illustrating example components forswitching a filter in or out of the line during a network upgrade.System 1500 comprises a switch and a filter. The left side of each ofFIGS. 11A and 11B illustrates a symbolic representation of the state ofthe filter (in or out of the line). The right side illustrates actualexample components for switching of the filter in or out of the line.The switch of system 1500 may comprise any suitable switch and isoperable to switch the filter in or out of the line. The filter coupledto the switch may include any of the filters discussed above, such as,for example, filters 428, 528, or 628 of networks 400, 500, or 600,respectively (which may be removed from a PSPON after an upgrade).Coupling switches to the filters described allows for easy connection ordisconnection of the filters from the network, avoiding contamination ofconnectors and providing a fast upgrade of the network. So, for example,in an upgrade of a PSPON, if the PSPON transmitter will no longer beused (or will be used only occasionally), the filter (such as, forexample, filter 528 of network 500) coupling the PSPON transmitter tothe network may seamlessly be taken out of the line using a switch asillustrated in FIG. 11B. Due to the switch, no cut need be made to thefiber coupling the filter to the network, avoiding disruptions inservice and contamination of connectors.

FIGS. 12A, 12B, 12C and 12D are diagrams illustrating example componentsfor switching filters in or out of the line during a network upgrade.More specifically, FIGS. 12A-12D illustrate an example filter system1600 that may be added to an RN of an upgradeable PSPON and remain inHPON (such as WS-HPON) and WDMPON upgrades. As an example, filter system1600 may be used in place of filter 442 of network 400, filter 542 ofnetwork 500, and filter 642 of network 600. By having a filter systemthat is operable to suitably filter traffic for networks 400, 500, and600, network operators need not cut fiber, replace components, or riskcontamination or service disruption at this point of the network. Thus,filter system 1600 may enhance the upgradeability of PSPON 400 byanticipating upgrades to the network (such as those described inconjunction with networks 500 and 600). As in FIGS. 11A and 11B, theleft side of each of FIGS. 12A, 12B, 12C and 12D illustrates a symbolicrepresentation of the state of the filter(s) (in or out of the line).The right side illustrates actual example components for switching ofthe filter(s) in or out of the line.

Filter system 1600 comprises filters 1610, 1620, and 1630 coupled tocorresponding switches. Filter 1610 may comprise any suitable filteroperable to pass downstream WDM traffic (for example, in theabove-described WS-HPON, traffic in λ₁-λ₈) and broadcast traffic (forexample, in the above-described WS-HPON, traffic in λ_(d) and λ_(v)) tofilter 1620. Filter 1610 is further operable to receive traffic inupstream wavelength λ_(u) from filter 1630 and direct it to the OLT.Filter 1620 may comprise any suitable filter operable to receive thedownstream WDM traffic and broadcast traffic from filter 1610, directthe broadcast traffic toward filter 1630, and pass the WDM traffictoward a wavelength router. Filter 1630 may comprise any suitable filterand is operable to receive the downstream broadcast traffic from filter1620 and direct the broadcast traffic toward the power splitter. Filter1630 is further operable to receive upstream traffic in λ_(u) and passit to filter 1610.

FIG. 12A illustrates the components and operation of filter system 1600in a PSPON such as upgradeable PSPON 400. In operation, filter 1610receives downstream broadcast traffic in λ_(d) and λ_(v) and passesλ_(d) and λ_(v) to filter 1620. Filter 1620 receives the downstreambroadcast traffic and directs it toward filter 1630. Filter 1630receives the downstream broadcast traffic and directs it toward thepower splitter. In the upstream direction, filter 1630 receives upstreamtraffic in time-shared λ_(u) and passes it to filter 1610. Filter 1610receives the upstream traffic and directs it toward the OLT.

FIG. 12B illustrates the components and operation of filter system 1600of FIG. 12A after an upgrade to a WS-HPON (such as WS-HPON 500 of FIG.6). The upgrade includes the addition of four new transmitters whichtransmit downstream WDM traffic in λ₁-λ₄. Furthermore, in this upgrade,downstream traffic is being broadcast only in λ_(v). Upstream trafficcontinues to be transmitted in time-shared λ_(u). In operation, in thedownstream direction, filter 1610 receives WDM traffic in λ₁-λ₄ andbroadcast traffic in λ_(v), and passes the traffic in λ₁-λ₄ and λ_(v) tofilter 1620. Filter 1620 receives the traffic in λ₁-λ₄ and λ_(v),directs the traffic in λ_(v) toward filter 1630, and allows the trafficin λ₁-λ₄ to pass to the wavelength router. Filter 1630 receives thetraffic in λ_(v) and directs it toward the power splitter. In theupstream direction, filter 1630 receives the traffic in λ_(u) and allowsit to pass to filter 1610. Filter 1610 receives the traffic in λ_(u) anddirects it toward the OLT. Thus, an upgrade from PSPON to WS-HPON may bemade without replacing components at this point in the network, avoidingservice disruption and contamination of components.

FIG. 12C illustrates the components and operation of filter system 1600of FIG. 12A after a further upgrade to the network (such as the upgraderepresented by WS-HPON 600 of FIG. 7). The upgrade includes the additionof four new transmitters transmitting downstream WDM traffic in λ₅-λ₈(which may be in and around the band of λ_(v)). Furthermore, in thisexample upgrade, downstream traffic is no longer being broadcast.Upstream traffic continues to be transmitted in time-shared λ_(u). Inoperation, in the downstream direction, filter 1610 receives the trafficin λ₁-λ₈ and passes it to the wavelength router. Filter 1620 is switchedout of the line since it no longer forwards broadcast downstream trafficto the power splitter. In the upstream direction, filter 1630 receivestraffic in λ_(u) and allows it to pass to filter 1810. Filter 1610receives the traffic in λ_(u) and directs it toward the OLT. Again, anupgrade to the network may be made without replacing components at thispoint in the network, avoiding service disruption and contamination ofcomponents.

FIG. 12D illustrates the components and operation of filter system 1600of FIG. 12A after a further upgrade to a WDMPON (or to a WS-HPON withupstream WDM capabilities). The upgrade includes the addition of newtransmitters at the OLT and at the ONUs such that each ONU is associatedwith a dedicated downstream and upstream wavelength. Thus, traffic inλ₁λ_(n) is transmitted in the downstream direction, and traffic inλ₁-λ_(n) is transmitted in the downstream direction (where n representsthe number of ONUs). λ₁-λ_(n) in the downstream direction may be thesame (or different) as λ₁-λ_(n) in the upstream direction. In operation,the traffic in λ₁-λ_(n) travels from the OLT to the wavelength router inthe downstream direction, and the traffic in λ₁-λ_(n) travels from thewavelength router to the OLT in the upstream direction. As illustrated,filter system 1600 may be switched out of the network, as the filters nolonger direct traffic to or from the power splitter. Thus, the line fromthe power splitter to filter system 1600 may be suitably terminated,such as, for example, using a small radius fiber loop.

Modifications, additions, or omissions may be made to the filter systems1500 and 1600 described without departing from the scope of theinvention. The components of the filter systems 1500 and 1600 describedmay be integrated or separated according to particular needs. Moreover,the operations of the filter systems 1500 and 1600 described may beperformed by more, fewer, or other components.

Although the present invention has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present invention encompasssuch changes and modifications as fall within the scope of the appendedclaims.

1. An upgradeable optical network, comprising: an upstream terminaloperable to forward a downstream signal comprising traffic in a firstwavelength, the traffic in the first wavelength transmitted by a firsttransmitter at the upstream terminal; and a distribution nodecommunicatively coupled to the upstream terminal, the distribution node:operable to communicate the downstream signal from the upstream terminalto a first plurality and a second plurality of downstream terminals;comprising a primary power splitter configured to create a plurality ofcopies of the downstream signal for communication to the downstreamterminals, the primary power splitter further configured to receive anupstream signal comprising at least traffic in a fourth wavelength fromthe downstream terminals, wherein the downstream terminals share thefourth wavelength for transmission of upstream traffic; and configuredto facilitate the addition of a wavelength router at a section of thedistribution node such that: the wavelength router may be added to thedistribution node without interrupting the communication of trafficthrough the network; and the addition of the wavelength router allowsfor: routing of traffic in a second wavelength transmitted by a secondtransmitter to be added at the upstream terminal, the traffic destinedfor the first plurality of downstream terminals; routing of traffic in athird wavelength transmitted by a third transmitter to be added at theupstream terminal, the traffic destined for the second plurality ofdownstream terminals; and the downstream terminals to continue to sharethe fourth wavelength for transmission of upstream traffic and theprimary power splitter to continue to receive the traffic in the fourthwavelength from the downstream terminals.
 2. The network of claim 1,wherein the upstream terminal comprises a switch operable to switch thefirst transmitter such that the first transmitter no longer transmitstraffic over the network.
 3. The network of claim 1, wherein thedistribution node comprises a filter operable to forward the traffic inthe first wavelength in a first direction toward the primary powersplitter, the filter further operable to forward the traffic in thesecond and third wavelengths in a second direction toward the section ofthe distribution node for the wavelength router.
 4. The network of claim1, wherein the downstream signal further comprises traffic in a fourthwavelength, the traffic in the fourth wavelength comprising broadcasttraffic, the traffic in the fourth wavelength transmitted by a fourthtransmitter at the upstream terminal.
 5. The network of claim 4, whereinthe upstream terminal is configured to facilitate the addition of thesecond and third transmitters by comprising a multiplexer, themultiplexer coupled to the fourth transmitter and operable to combinetraffic transmitted by the second, third, and fourth transmitters afterthe addition of the second and third transmitters.
 6. The system ofclaim 1, wherein the upstream terminal comprises the second and thirdtransmitters, a first filter, a second filter, and a cyclic multiplexer,wherein: the second transmitter is coupled to a first port of the cyclicmultiplexer through the first filter; the third transmitter is coupledto a second port of the cyclic multiplexer through the second filter;and the upstream terminal is configured to facilitate the addition of afourth transmitter for transmission of downstream traffic in a fourthwavelength and a fifth transmitter for transmission of downstreamtraffic in a fifth wavelength such that: the fourth transmitter would becoupled to the first port of the cyclic multiplexer through the firstfilter; the fifth transmitter would be coupled to the second port of thecyclic multiplexer through the second filter; the first filter wouldcombine the traffic in the second and fourth wavelengths; and the secondfilter would combine the traffic in the third and fifth wavelengths. 7.The system of claim 1, wherein the wavelength router comprises amultiplexer.
 8. The system of claim 7, wherein the multiplexer comprisesa cyclic multiplexer.
 9. The system of claim 1, wherein the wavelengthrouter comprises a plurality of cascaded filters.
 10. The system ofclaim 1, wherein the distribution node comprises a plurality ofsecondary power splitters, the plurality of secondary power splitterscomprising a first secondary power splitter and a second secondary powersplitter, the first secondary power splitter configured to be operableto: receive a first copy from the primary power splitter; forward thefirst copy to the first plurality of downstream terminals; and after thewavelength router is added at the distribution node, receive the trafficin the second wavelength from the wavelength router and forward thetraffic in the second wavelength to the first plurality of downstreamterminals; and the second secondary power splitter configured to beoperable to: receive a second copy from the primary power splitter;forward the second copy to the second plurality of downstream terminals;and after the wavelength router is added to the distribution node,receive the traffic in the third wavelength from the wavelength routerand forward the traffic in the third wavelength to the second pluralityof downstream terminals.
 11. A method for upgrading an optical networkthat comprises an upstream terminal, a distribution node, and two ormore downstream terminals, the upstream terminal comprising a firsttransmitter transmitting downstream traffic in a first wavelength, andthe distribution node comprising a primary power splitter configured tosplit the traffic in the first wavelength into a plurality of copies andforward the copies to a first plurality and a second plurality ofdownstream terminals, wherein the upstream terminal further comprises areceiver receiving upstream traffic in a fourth wavelength that isshared by the downstream terminals for transmission of upstream traffic,the method comprising: adding a first plurality of transmitters at theupstream terminal, a first transmitter of the first plurality oftransmitters operable to transmit downstream traffic in a secondwavelength, a second transmitter of the first plurality of transmittersoperable to transmit downstream traffic in a third wavelength; adding amultiplexer at the upstream terminal, the multiplexer operable tocombine the downstream traffic in the second and third wavelengths; andadding a wavelength router to the distribution node, the wavelengthrouter operable to route the downstream traffic in the second wavelengthto the first plurality of downstream terminals and to route thedownstream traffic in the third wavelength to the second plurality ofdownstream terminals.
 12. The method of claim 11, further comprisingadding a plurality of combining elements to the distribution nodedownstream of the wavelength router, the combining elements comprising:a first combining element operable to combine the traffic in the secondwavelength from the wavelength router and a first copy of the firstwavelength from the power splitter; and a second combining elementoperable to combine the traffic in the third wavelength from thewavelength router and a second copy of the first wavelength from thepower splitter.
 13. The method of claim 12, wherein at least one of thefirst and second combining elements is selected from the groupconsisting of a filter, a coupler, and a combination thereof.
 14. Themethod of claim 11, further comprising: adding a second plurality oftransmitters to the upstream terminal, a first transmitter of the secondplurality of transmitters operable to transmit downstream traffic in afourth wavelength, a second transmitter of the second plurality oftransmitters operable to transmit downstream traffic in a fifthwavelength; and using the multiplexer of the upstream terminal tocombine the downstream traffic in the second, third, fourth, and fifthwavelengths.
 15. The method of claim 14, wherein the wavelength routeris operable to: route the traffic in the fourth wavelength to the firstplurality of downstream terminals; and route the traffic in the fifthwavelength to the second plurality of downstream terminals.
 16. Themethod of claim 14, wherein the wavelength router is operable to: routethe traffic in the fourth wavelength to a third plurality of downstreamterminals; and route the traffic in the fifth wavelength to a fourthplurality of downstream terminals.
 17. The method of claim 11, furthercomprising terminating transmission by the first transmitter.
 18. Themethod of claim 11, wherein the multiplexer of the upstream terminalcomprises a cyclic multiplexer.
 19. The method of claim 11, wherein thewavelength router comprises a multiplexer.
 20. The method of claim 19,wherein the multiplexer comprises a cyclic multiplexer.
 21. The methodof claim 11, wherein the wavelength router comprises a plurality ofcascaded filters.
 22. The method of claim 11, wherein: the upstreamterminal comprises a second transmitter transmitting downstream trafficin a fourth wavelength; and the primary power splitter of thedistribution node is configured to split the traffic in the fourthwavelength into a plurality of copies and forward the copies to thefirst plurality and the second plurality of downstream terminals.